centripetal force water experiment

• Earth Science
• Physics & Engineering
• Science Kits
• Microscopes
• Science Curriculum and Kits
• About Home Science Tools

Science Projects > Physics & Engineering Projects > Centripetal Force  

Centripetal Force

What you need:.

• Small plastic bucket
• Sturdy string
• Outdoor place that is OK to get wet

What You Do:

1. Tie the string securely around the handle of a plastic bucket. Use a plastic bucket rather than metal so it is not heavy.

2. Pour a glass full of water into the bucket. (Again, don’t add too much water so it is not too heavy.)

3. Hold the string so that the bucket is about level with your knees. Adjust length of string as needed.

4. Spin the bucket of water over your head in a vertical (up and down) direction. Make sure you spin the bucket fast enough so it stays in a circular path. Does the water spill out?

What Happened:

It seems as if the water in the bucket is defying gravity, but is it really?

No. Gravity – the force pulling down on everything – is still at work even when the bucket and water are above your head. The water’s inertia wants to keep the water traveling in a straight path, but gravity is acting on the water, causing it to fall in a downward path that will eventually hit the earth. However, while the water is falling, the bucket is falling with it, catching the water. What keeps the bucket and water moving in a nice circular path that doesn’t get wet or messy is the string. The string acts as the centripetal force that pulls the bucket and water into the center and keeps them from following their paths of inertia, giving the illusion that centrifugal force is pulling the water away from the center. But be careful. In order for the bucket to keep falling with the water, the bucket must travel fast enough to keep up with the water. If you spin the bucket too slowly, the water will fall out and you will get wet.

Physics & Engineering

Welcome! Read other Physics & Engineering related articles or explore our Resource Center, which consists of hundreds of free science articles!

Shop for Physics Supplies!

Home Science Tools offers a wide variety of Physics products and kits. Find physics & engineering tools, equipment, STEM kits & more for kids and adults.

Related Articles

Homopolar Motor – Make a Spinning Wire Sculpture

In this experiment, we will make a homopolar motor! To make a simple motor (homopolar motor) that doubles as a work of art you will need three things – a battery, magnet, and wire. Use one of our neodymium magnets to power the spinning wire motor. What You Will Need:...

Solar Energy Matching Game

Print out this page on a sheet of heavy paper or cardstock. Kids can color the pictures and cut out the squares to make a matching game. Half of the squares show a way to use solar energy as an alternative to the picture shown on the other squares. Place all the...

Simple Spring Break Science Projects

Spring break is here! What will you do with your time off? Perhaps you're looking forward to a family vacation, or a few days of down time at home. Either way, find a quick and easy project that's sure to put a sparkle in the eye of any science lover, or win over a...

Sink or Float Worksheet

Use the Sink or Float Worksheet with the "Sink or Float?" science project to encourage kids to make predictions, perform tests, and record their results. This project is perfect for indoor discovery - especially in colder months! Put a towel under a large container of...

Physics Science Fair Projects

Find physics science fair project ideas about magnetism, electricity, energy and solar power, and more.

JOIN OUR COMMUNITY

Get project ideas and special offers delivered to your inbox.

Centripetal Force on a Spinning Cup

Centripetal Force on a Spinning Cup   principles in circular motion by having a cup of water seemingly defy gravity. The demonstration is used to explain centripetal acceleration, and a central force. While this demonstration takes some practice, it is one of the most fun and rewarding demonstration we have.

• Back To Mechanics

Physics Behind The Demo

• Presenting The Demo
• This is the platform on which the cup will sit while being spun in circles
• Over time, the knots in the string will slip, and the platform will no longer be level. This can be fixed by untying and releveling the platform. Since this is quite tedious, the TA or instructor should be informed as soon as the platform seems to be slipping so that it can be repaired without a time constraint.
• Any transparent cup will work, but a cheap plastic one is ideal because the cup will eventually be launched across the room on accident.
• Be sure to locate a source of water before starting your presentations.
• If you think you might spill water, you can practice with the tennis ball in the cup instead of water. 
• This demo is scary at first, but with practice can become second nature.

Back To Top

In simplest terms, the water cup is undergoing uniform circular motion.

Uniform circular motion can be described as the motion of an object in a circle at a constant speed. As an object moves in a circle, it is constantly changing its direction. At all instances, the object is moving tangent to the circle. Since the direction of the velocity vector is the same as the direction of the object's motion, the velocity vector is directed tangent to the circle as well.

An object moving in a circle is accelerating. Accelerating objects are objects that are changing their velocity; either the speed or the direction. An object undergoing uniform circular motion is moving with a constant speed. Nonetheless, it is accelerating due to its change in direction. The direction of the acceleration is inwards and the object feels a net force pointing towards the center of teh circle.

The net force is an inward or “centripetal force”. Without such an inward force, an object would continue in a straight line, never changing its direction. Yet, with the inward net force directed perpendicular to the velocity vector, the object is always changing its direction and undergoing an inward acceleration.

To relate to the demo, as the water spins around, the centripetal force pushes the cup inward to the center of the circle and the water does not have time to accelerate downward. This is similar to the force that keeps you in your seat when doing a loop on a roller coaster. The water wants to fly off from the circle, but the bucket gets in the way and keeps it in place. This is the same effect you feel when you go around a tight corner in the car and get squished against the door or the force that keeps you in your seat when doing a loop on a roller coaster.

Ultimately, the force that is accelerating the platform and water cup toward the center is the tension in the string. The tension required to maintain circular motion is described in the following equation:

$$ \mathbf{F}_{tension}={{mv^2}\over{r}}$$

The Tension force "$ \LARGE\mathbf{F}_{tension} $" is the centripetal force. This equation shows that you must increase the inward force if you increase the velocity, or decrease the radius.

Performing The Demo

Each tab contains a different method for presenting this demo as written by a former outreach student.

Before Spinning the Cup

• Check that you have enough room around you so that you will not hit anyone or anything with the platform.
• Place the cup filled with water in the center of the board and check that the board is level.
• Begin by slowly swinging the platform back and forth to get a feel for it.
• When you are comfortable, swing the platform quickly enough to get it going around in the circle.
• When changing the orientation of the swing or the speed, do so slowly and deliberately. The more changes in acceleration there are, the more likely it is that the water will spill.
• When you are ready to stop, do so when the platform is on the bottom of its swing.
• Take a step in the same direction the platform is moving to give it a long enough time to slow down.

Explaining the Demo

Sample "script"

“Alright so what I have here is a water cup that I’m going to spin around in a circle above my head. What do you guys think is going to happen?”

“Alright so let’s see if you guys are right.”

(Demonstrator proceeds to spin the water cup until it is over his head, or on his/her side if they aren’t comfortable with going overhead)

(As cup is spinning) “As you guys can see the water isn’t coming out. Can anyone tell me why?”

“Alright all good Ideas (stops spinning) Alright well the reason the water stays in the cup is what’s called centripetal force. All of you guys have experienced that same force in some way.”

(Awes of disbelief)

“So how many of you guys have ridden a roller-coaster that goes upside down?”

(Hands raise)

“Alright, have you guys ever wondered why you haven’t fallen out while you’re upside down? (Silence momentarily) Like I said it’s due to the centripetal force of roller-coaster. “

“Now all of you guys haven’t ridden roller-coasters, but all have you have ridden in cars right?” (Yeses)

“Alright, how many of your parents have made a really fast turn and you felt yourself being pulled to the side of the car? (All think)”

“That’s the same thing, centripetal force. While the car is changing direction you want to keep going straight so you hit the door. “

“How many of you guys liked this demonstration? Alright that’s what I like to hear. Next we have……”

Back to Top

• Wrap the string around your finger a couple of times to make sure you have a good grip on it.
• Practice with a tennis ball in the cup before doing it with water.
• Don't be afraid of the demo, the worst that can happen is something gets wet. Kids are messy, everything in a school has been wet at some point.

The Spinning Penny

This activity is an amazingly simple display of centripetal force (and annoying sounds) right at your fingertips.

Print this Experiment

Warning: The Spinning Penny Balloon is known to be addicting! Once you try it, you’ll find out too late that it’s habit forming and totally cool. Left untreated, you’ll be test-spinning everything in sight. It’s the simplicity of the setup and the wide variety of different sounds you can generate that make it so hard to put down. Besides, you use some very serious science to annoy the heck out of those around you.

Experiment Videos

Here's What You'll Need

Round balloons of different sizes and colors, variety of coins of different sizes, various hex nuts or small round objects with bumpy sides, let's try it.

Slip a coin or a hex nut (a round object) through the mouth of a balloon. The object needs to go all the way into the balloon so that there’s no danger of it being inhaled as you blow up the balloon. (Clear latex balloons allow you see what’s going on inside but any size and color of ball-shaped balloons will work.)

Inflate the balloon to a comfortable size. That means a size you can easily hold and control.

Tie off the balloon with a simple overhand knot and you’re ready to go.

Grip the balloon at the stem end with your open writing hand. The neck of the balloon is in your palm and your fingers and thumb extend down the sides. The object will be on the bottom, inside the balloon.

While holding the balloon this way, move it in a rapid, circular motion. The object may bounce around at first, but it will soon begin to zoom around the inside wall of the balloon. The best path for the object is about parallel to the floor.

Once the spinning begins, use your other hand to stabilize the balloon, if needed. The object could continue to spin for 30 seconds or more! If it has bumpy sides, you’ll be hearing a lot of noise, too.

As the speed of the object changes, listen to the changes in the sounds it makes and its location inside the balloon.

Stop spinning the balloon and let the object come to a complete stop again on the bottom of the balloon. This activity screams, “Take it further!”

How Does It Work

The Spinning Penny is scientific poetry in motion. To understand how and why it works, you have to look at the forces that are acting on the penny (object). Of course, you add the energy to get things started and to keep it going with the force you put in to swirling the balloon. The shape of the balloon forces the penny (or any object in it) to move in a circular path – otherwise, it would continue to move off in a straight line. Another force to consider is friction between the balloon and the object. While there’s very little friction between the edge of the object and the balloon, it is still there and doing its job. Friction finally causes the penny to slow down and to stop. It gets lower and lower inside the balloon because of our old friend, gravity , too. While the object’s mass stays the same, its speed drops because of friction against the balloon and moves lower because of gravity.

The real force in action here is called centripetal force , which means center-seeking. This is a force that is always directed toward the center of circular movement and is actually responsible for keeping the penny moving in a circle. Inside the balloon, it’s the wall of the balloon that causes this to happen. Out in the solar system, it’s the pull of the Sun’s gravity that keeps Earth in its circular orbital path.

Hopefully, you used a bumpy-sided object (like a hex nut) and heard some crazy noises. It sounded like the balloon was screaming! The rough sides of the object don’t roll smoothly over the balloon wall. Instead, it bounces from one flat edge to another. This bouncing accounts accounts for the noise and the change in pitch comes from the speed of the object: higher pitch = faster speed and lower pitch = slower speed.

Take It Further

Use different sized coins and objects to compare how long it takes for them to stop spinning after you stop swirling the balloon. What can you say about how the size or shape of an object changes the outcome?

What happens to your results when you use balloons of increasing diameters? It’s a little tough to handle larger balloons but the results are worth it. If you can get your hands on a really big latex balloon (visit a local party-supply store), then you can see the results of your testing. The size, mass, and speed of the objects spinning inside the balloon will all influence the results you discover.

Related Experiments

Spinning Glasses of Water - Centripetal Force Board

When you swing a bucket of water over your head, you probably expect a big, wet rush of water to soak you as the bucket […]

Newton's Inertia Beads - The Chain Fountain

A long chain of beads appears to siphon itself from a container as the beads twist and turn in a most mesmerizing dance. As you’ll […]

Inertia Ring

The object of this activity is to get the hex nut to drop into the plastic bottle. Simple enough but there will be something in the […]

Spinning Match - Table Trick

When you cautiously balance a matchstick on the rim of a coin that has also been precariously balanced onto another coin, it might sound like […]

Marble Gravitron

Recently, the Spangler Labs Research Team took a trip to an amusement park. (It wasn’t for fun, it was for work but it was fun.) […]

Browse more experiments by concept:

• Science Notes Posts
• Contact Science Notes
• Todd Helmenstine Biography
• Anne Helmenstine Biography
• Free Printable Periodic Tables (PDF and PNG)
• Periodic Table Wallpapers
• Interactive Periodic Table
• Periodic Table Posters
• Science Experiments for Kids
• How to Grow Crystals
• Chemistry Projects
• Fire and Flames Projects
• Holiday Science
• Chemistry Problems With Answers
• Physics Problems
• Unit Conversion Example Problems
• Chemistry Worksheets
• Biology Worksheets
• Periodic Table Worksheets
• Physical Science Worksheets
• Science Lab Worksheets
• My Amazon Books

Centripetal Force Definition, Examples, and Formula

Centripetal force is a fundamental concept in physics, referring to the force that acts on an object moving in a circular path and is directed towards the center around which the object is moving. This force maintains the circular motion of the object, preventing it from moving off its path due to inertia.

• Centripetal force is the force that acts toward the center of a circular path.
• The force is always perpendicular to the direction of movement.
• The formula for centripetal force is F c = mv 2 /r.
• The force pushes or pulls an object toward the center of rotation, for example, in planets orbiting the Sun, turning a car, or spinning a ball on a string.

Historical Background and Word Origin

The concept of centripetal force dates back to the early scientific explorations of motion and gravity. The term ‘centripetal’ originates from the Latin words ‘centrum’ meaning center and ‘petere’ meaning to seek. Sir Isaac Newton term popularized the term in the scientific community in the 17th century, particularly through his work “Principia Mathematica.”

Units of Centripetal Force

The unit of centripetal force, like all forces in physics, is the Newton (N) in the International System of Units (SI). This derived unit gets its name in recognition of Sir Isaac Newton’s work in classical mechanics.

Importance of Understanding Centripetal Force

Understanding centripetal force is crucial in various fields, from engineering to astronomy. It helps in analyzing the motion of objects following a curved path, ranging from electrons in a magnetic field to planets orbiting a star. It also plays a vital role in designing vehicles, amusement park rides, and understanding celestial mechanics.

Examples of Centripetal Force

• Planetary Orbits : Planets orbiting the Sun stay in their elliptical paths due to the centripetal force exerted by the Sun’s gravitational pull.
• Vehicle Turns : When a car makes a turn, the centripetal force comes from the friction between the car’s tires and the road.
• Roller Coasters : The loops in roller coasters are classic examples where centripetal force is at work, keeping the cars on their tracks.
• Satellites : Artificial satellites orbiting the Earth experience centripetal force due to Earth’s gravity.
• Spinning Objects: When spinning a ball on a string, tension on the string pulls the ball toward the center.

How Centripetal Force Works

Centripetal force is not a fundamental force. Rather, it is the net force that makes an object move in a circular path. It has several sources:

• In astronomical contexts, the gravitational attraction between two bodies, such as a planet and its moon or a star and its orbiting planet, acts as the centripetal force. This force keeps the orbiting body in a stable, typically elliptical, orbit.
• On Earth, friction often provides the centripetal force needed for circular motion. For example, when a car turns a corner, the friction between the tires and the road provides the necessary centripetal force to keep the car on its curved path.
• In scenarios involving strings or ropes, such as a ball swung in a circle on a string or a tetherball, the tension in the string or rope provides the centripetal force. This force acts along the string, pulling the object towards the center of the circular path.
• In roller coasters or when a vehicle goes over a hill, the normal force exerted by the track or the road acts as the centripetal force. This is especially evident in roller coaster loops where the track’s structure exerts an inward normal force on the carts.
• In certain physical and engineering applications, magnetic forces provide centripetal force. For instance, in a cyclotron (a type of particle accelerator), charged particles spiral outward in a magnetic field. The magnetic force acts perpendicular to their velocity, providing the centripetal force that keeps them in a circular path.
• In atomic and subatomic scales, electrostatic forces (like the force between electrons and the nucleus) act as centripetal forces. For instance, in the Rutherford or Bohr models of the atom, the electrostatic attraction between the positively charged nucleus and the negatively charged electrons provides the centripetal force that keeps the electrons in their orbits.

Formula and Derivation from Newton’s Laws

The two key formulas are for centripetal acceleration (a c ) and centripetal force (F c ):

• Where v is the velocity of the object and r is the radius of the circular path.
• Here, m is the mass of the object.
• These formulas derive from Newton’s second law of motion : F=ma.

Centripetal vs. Centrifugal Force

Centripetal force is the real force that acts towards the center of the circle. On the other hand, centrifugal force is a perceived force that appears to act outward on an object when viewed from a rotating frame of reference. It is not an actual force but a result of the inertia of an object moving in a curved path. Centripetal and centrifugal forces are equal in magnitude, but opposite in direction.

Where Centripetal Force Is Greatest

The magnitude of centripetal force depends on the object’s velocity and the radius of the circular path. It is greatest when the speed is highest or at the smallest radius of curvature.

Centripetal Force and Velocity

Centripetal force is proportional to the square of the velocity of the object. As the velocity increases, the required centripetal force to maintain the circular motion increases quadratically. For example, doubling the speed of an object requires four times the centripetal force to keep it in circular motion.

Effect of Radius on Centripetal Force

Centripetal force is inversely proportional to the radius of the circular path. As the radius increases, the required centripetal force for maintaining circular motion decreases.

Practical Applications

Centripetal force plays a key role in many practical calculations:

• Astronomy : Understanding the motion of celestial bodies.
• Engineering : Design of roads, vehicles, and amusement park rides.
• Centrifuges : Used in medical and scientific laboratories.
• Athletics : Techniques in sports like hammer throw and discus.
• Communications : Satellite orbits for global communication networks.

In conclusion, centripetal force is a pivotal concept in physics, with wide-ranging applications across multiple disciplines. Its understanding allows us to comprehend and predict the motion of objects in circular paths, contributing significantly to technological and scientific advancements.

Frequently Asked Questions (FAQ)

Here are some frequently asked questions that also address common misconceptions about centripetal force:

• Centripetal force does not affect the speed (magnitude of velocity) of an object in circular motion; instead, it changes the direction of the velocity. The speed remains constant unless an additional external force is applied.
• No, centripetal force is not always gravitational. It is any force that keeps an object moving in a circular path and can arise from various sources, such as tension, friction, gravitational attraction, or magnetic forces.
• According to Newton’s third law of motion, every action has an equal and opposite reaction. In the context of centripetal force, while the centripetal force acts towards the center of the circular path, the object in motion exerts an equal and opposite force (often perceived as the centrifugal force in a rotating reference frame) away from the center.
• If the centripetal force is removed suddenly, the object no longer follows a circular path. Instead, it moves off in a straight line tangential to the circular path at the point of release, in accordance with Newton’s first law of inertia.
• Centripetal force itself is not negative; it is always directed towards the center of the circular path. The concept of negative force in this context doesn’t apply because the direction of the force (towards the center) is what defines it as centripetal.
• Beiser, Arthur (2004). Schaum’s Outline of Applied Physics . New York: McGraw-Hill Professional. ISBN 978-0-07-142611-4.
• Hibbeler, Russell (2009). “Equations of Motion: Normal and tangential coordinates”. Engineering Mechanics: Dynamics (12th ed.). Prentice Hall. ISBN 978-0-13-607791-6.
• Serway, Raymond A.; Jewett, John W. (2004). Physics for Scientists and Engineers (6th ed.). Brooks/Cole. ISBN 978-0-534-40842-8.
• Stanford Encyclopedia of Philosophy (2007). “ Newton’s Philosophiae Naturalis Principia Mathematica “.
• Tipler, Paul; Mosca, Gene (2003). Physics for Scientists and Engineers (5th ed.). Macmillan. ISBN 978-0-7167-8339-8.

Related Posts

1-800-866-1360

To demonstrate the concept of centripetal force

• Plastic bucket

Put water in the bucket until it is three-quarters full. Make sure the bucket is not completely full!  Pick up the bucket by the handle. Starting with your arm at your side, turn it around in a sweeping motion. Make it move toward the sky, and then to the ground, in a windmill motion. You should make a complete circle with your arm. Be consistent with the speed and rotation.

The spinning bucket of water is a demonstration of centripetal force, the inward force acting on a rotating object. The water is falling at the same rate as the bucket is rotating. If you slow down the rotation of the bucket, you will get wet. This is because the water is falling faster than the rotating bucket. Practice this outside!  

• News/Resources

Creative3, LLC 1-800-866-1360 (phone and fax) 2236 E. Spring Hill Road Springfield, MO 65804

The Centripetal Force Requirement

• Speed and Velocity
• Centripetal Force Requirement
• The Forbidden F-Word
• Mathematics of Circular Motion

To understand the importance of a centripetal force, it is important to have a sturdy understanding of the Newton's first law of motion - the law of inertia . The law of inertia states that ...

... objects in motion tend to stay in motion with the same speed and the same direction unless acted upon by an unbalanced force.

According to Newton's first law of motion, it is the natural tendency of all moving objects to continue in motion in the same direction that they are moving ... unless some form of unbalanced force acts upon the object to deviate its motion from its straight-line path. Moving objects will tend to naturally travel in straight lines; an unbalanced force is only required to cause it to turn. Thus, the presence of an unbalanced force is required for objects to move in circles.

Inertia, Force and Acceleration for an Automobile Passenger

The idea expressed by Newton's law of inertia should not be surprising to us. We experience this phenomenon of inertia nearly everyday when we drive our automobile. For example, imagine that you are a passenger in a car at a traffic light. The light turns green and the driver accelerates from rest. The car begins to accelerate forward, yet relative to the seat which you are on your body begins to lean backwards. Your body being at rest tends to stay at rest. This is one aspect of the law of inertia - "objects at rest tend to stay at rest." As the wheels of the car spin to generate a forward force upon the car and cause a forward acceleration, your body tends to stay in place. It certainly might seem to you as though your body were experiencing a backwards force causing it to accelerate backwards. Yet you would have a difficult time identifying such a backwards force on your body. Indeed there isn't one. The feeling of being thrown backwards is merely the tendency of your body to resist the acceleration and to remain in its state of rest. The car is accelerating out from under your body, leaving you with the false feeling of being pushed backwards.

Now imagine that you are in the same car moving along at a constant speed approaching a stoplight. The driver applies the brakes, the wheels of the car lock, and the car begins to skid to a stop. There is a backwards force upon the forward moving car and subsequently a backwards acceleration on the car. However, your body, being in motion, tends to continue in motion while the car is skidding to a stop. It certainly might seem to you as though your body were experiencing a forwards force causing it to accelerate forwards. Yet you would once more have a difficult time identifying such a forwards force on your body. Indeed there is no physical object accelerating you forwards. The feeling of being thrown forwards is merely the tendency of your body to resist the deceleration and to remain in its state of forward motion. This is the second aspect of Newton's law of inertia - "an object in motion tends to stay in motion with the same speed and in the same direction... ." The unbalanced force acting upon the car causes the car to slow down while your body continues in its forward motion. You are once more left with the false feeling of being pushed in a direction which is opposite your acceleration.

These two driving scenarios are summarized by the following graphic.

In each case - the car starting from rest and the moving car braking to a stop - the direction which the passengers lean is opposite the direction of the acceleration. This is merely the result of the passenger's inertia - the tendency to resist acceleration. The passenger's lean is not an acceleration in itself but rather the tendency to maintain the state of motion while the car does the acceleration. The tendency of a passenger's body to maintain its state of rest or motion while the surroundings (the car) accelerate is often misconstrued as an acceleration. This becomes particularly problematic when we consider the third possible inertia experience of a passenger in a moving automobile - the left hand turn.

The Centripetal Force and Direction Change

Any object moving in a circle (or along a circular path) experiences a centripetal force . That is, there is some physical force pushing or pulling the object towards the center of the circle. This is the centripetal force requirement. The word centripetal is merely an adjective used to describe the direction of the force. We are not introducing a new type of force but rather describing the direction of the net force acting upon the object that moves in the circle. Whatever the object, if it moves in a circle, there is some force acting upon it to cause it to deviate from its straight-line path, accelerate inwards and move along a circular path. Three such examples of centripetal force are shown below.

The centripetal force for uniform circular motion alters the direction of the object without altering its speed. The idea that an unbalanced force can change the direction of the velocity vector but not its magnitude may seem a bit strange. How could that be? There are a number of ways to approach this question. One approach involves to analyze the motion from a work-energy standpoint. Recall from Unit 5 of The Physics Classroom that work is a force acting upon an object to cause a displacement . The amount of work done upon an object is found using the equation

where the Theta in the equation represents the angle between the force and the displacement. As the centripetal force acts upon an object moving in a circle at constant speed, the force always acts inward as the velocity of the object is directed tangent to the circle. This would mean that the force is always directed perpendicular to the direction that the object is being displaced. The angle Theta in the above equation is 90 degrees and the cosine of 90 degrees is 0. Thus, the work done by the centripetal force in the case of uniform circular motion is 0 Joules. Recall also from Unit 5 of The Physics Classroom that when no work is done upon an object by external forces, the total mechanical energy (potential energy plus kinetic energy) of the object remains constant. So if an object is moving in a horizontal circle at constant speed, the centripetal force does not do work and cannot alter the total mechanical energy of the object. For this reason, the kinetic energy and therefore, the speed of the object will remain constant. The force can indeed accelerate the object - by changing its direction - but it cannot change its speed. In fact, whenever the unbalanced centripetal force acts perpendicular to the direction of motion, the speed of the object will remain constant. For an unbalanced force to change the speed of the object, there would have to be a component of force in the direction of (or the opposite direction of) the motion of the object.

Applying Vector Components and Newton's Second Law

A second approach to this question of why the centripetal force causes a direction change but not a speed change involves vector components and Newton's second law . The following imaginary scenario will be used to help illustrate the point.

The examples above illustrate that a force is only capable of slowing down or speeding up an object when there is a component directed in the same direction or opposite direction as the motion of the object. In case e, the vertical force does not alter the horizontal motion. It is sometimes said that perpendicular components of motion are independent of each other. A vertical force cannot affect a horizontal motion.

To summarize, an object in uniform circular motion experiences an inward net force. This inward force is sometimes referred to as a centripetal force, where centripetal describes its direction. Without this centripetal force, an object could never alter its direction. The fact that the centripetal force is directed perpendicular to the tangential velocity means that the force can alter the direction of the object's velocity vector without altering its magnitude.

We Would Like to Suggest ...

Check Your Understanding

1. Which vector below represents the direction of the force vector when the object is located at point A on the circle?

2. Which vector below represents the direction of the force vector when the object is located at point C on the circle?

3. Which vector below represents the direction of the velocity vector when the object is located at point B on the circle?

4. Which vector below represents the direction of the velocity vector when the object is located at point C on the circle?

5. Which vector below represents the direction of the acceleration vector when the object is located at point B on the circle?

1. Answer = D

The force vector is directed inward to the circle; that would be downward when at point A

2.  Answer = B

The force vector is directed inwards; that would be up and to the right when the object is at point C.

3.  Answer = D

The velocity vector is directed tangent to the circle; that would be downwards when at point B.

4.   Answer = A

The velocity is directed tangentially; that would be upwards and leftwards when at point C.

5.  Answer = C

The acceleration would be directed inwards; that would be leftwards when the object is at point B.

6. Rex Things and Doris Locked are out on a date. Rex makes a rapid right-hand turn. Doris begins sliding across the vinyl seat (that Rex had waxed and polished beforehand) and collides with Rex. To break the awkwardness of the situation, Rex and Doris begin discussing the physics of the motion that was just experienced. Rex suggests that objects which move in a circle experience an outward force. Thus, as the turn was made, Doris experienced an outward force that pushed her towards Rex. Doris disagrees, arguing that objects that move in a circle experience an inward force. In this case, according to Doris, Rex traveled in a circle due to the force of his door pushing him inward. Doris did not travel in a circle since there was no force pushing her inward; she merely continued in a straight line until she collided with Rex. Who is correct? Argue one of these two positions.

Doris is correct.

When the turn is made, Doris continues in a straight-line path; this is Newton's first law of motion. Once Doris collides with Rex, there is then an unbalanced force capable of accelerating Doris towards the center center of the circle, causing the circular motion.

7. Kara Lott is practicing winter driving in the GBS parking lot. Kara turns the wheel to make a left-hand turn but her car continues in a straight line across the ice. Teacher A and Teacher B had viewed the phenomenon. Teacher A argues that the lack of a frictional force between the tires and the ice results in a balance of forces that keeps the car traveling in a straight line. Teacher B argues that the ice placed an outward force on the tire to balance the turning force and thus keep the car traveling in a straight line. Which teacher is (A or B) is the physics teacher? ______ Explain the fallacy in the other teacher's argument.

Teacher A is correct (and is hopefully the physics teacher).

A car turns in a circle due to the friction against its turned wheels. With wheels turned and no friction, there would be no circle. That is the problem in this situation.

• Newton's Second Law - Revisited

• Classical Physics

Water in a spinning bucket: a better explanation

• Thread starter fisico30
• Start date Feb 16, 2012
• Tags Explanation Spinning Water
• Feb 16, 2012
• New theory broadens phase transition exploration
• Physicists combine multiple Higgs boson pair studies and discover clues about the stability of the universe
• New method enhances X-ray microscopy for detecting tiny defects

This is supposed to be a better explanation than what exactly?  

I think the easiest way to explain this is if the bucket is accelerating downwards equal to or faster than 1 g, then the water will stay in the bucket because the bucket accelerates as fast or faster than gravity accelerates the water.  

• Feb 17, 2012

True. I have been trying to explain this common example to someone without knowledge of physics, forces, etc... But itself water would follow a parabolic path that would move it farther and out of the circular path. The role of the bucket is to constrain the water motion to a circular path. The inertia of water causes a contact force with the bottom of the bucket that points inward (part of the centripetal force). At critical velocity, the water at the top point of the trajectory is still falling down but not on our heads because the distance traveled downward is matched by the bucket which is always moving in sync with the water and catching it... If the bucket moves faster than the critical velocity the water would have the tendency, by inertia to move even farther out of the circular path. It is the bucket that keeps it from doing that (pressure at the bottom of the bucket). If the bottom of the bucket breaks water spills out radially... Simple problems but interesting to look at it in details, I think. The equation is simple (m v^2/r= n+mg) but does not shine enough light on the actual concept...  

fisico30, You have tried to put into your own words the phenomonem known as "Newton's Bucket" for a "better explanation of what I generally read". What do you think of Newton's description? “If a vessel, hung by a long cord, is so often turned about that the cord is strongly twisted, then filled with water, and held at rest together with the water; after, by the sudden action of another force, it is whirled about in the contrary way, and while the cord is untwisting itself, the vessel continues for some time this motion; the surface of the water will at first be plain, as before the vessel began to move; but the vessel by gradually communicating its motion to the water, will make it begin sensibly to revolve, and recede by little and little, and ascend to the sides of the vessel, forming itself into a concave figure...This ascent of the water shows its endeavour to recede from the axis of its motion; and the true and absolute circular motion of the water, which is here directly contrary to the relative, discovers itself, and may be measured by this endeavour. ... And therefore, this endeavour does not depend upon any translation of the water in respect to ambient bodies, nor can true circular motion be defined by such translation. ...; but relative motions...are altogether destitute of any real effect. ...It is indeed a matter of great difficulty to discover, and effectually to distinguish, the true motions of particular bodies from the apparent; because the parts of that immovable space in which these motions are performed, do by no means come under the observations of our senses.” — Isaac Newton; Principia, Book 1: Scholium http://en.wikipedia.org/wiki/Bucket_argument  

I don't believe that is what fisico30 is talking about. From his description, he is talking about swinging a bucket of water in a circle over his head. Newton was talking about spinning a bucket of water around its own axis.  

Oops! Excuse me, I misread the OP.  

• Feb 18, 2012

still interesting... so, physically, why does the water surface curves towards the edge as the bucket is rotating? Conceptually, how can this be explained in terms of the viscosity of water and the adhesion with the bucket's walls? thanks fisico30  

Partially but more importantly the water "particles" want to move in a straight line so they move to the side of the bucket which the stops it. The water "builds up" at the sides of the bucket.  

Related to Water in a spinning bucket: a better explanation

The phenomenon of water staying in a spinning bucket is called the "bucket of water" experiment or the "spinning bucket" experiment.

The water stays in the bucket due to centripetal force, which is the force that acts on an object moving in a circular path and keeps it from flying outwards.

The spinning motion creates a centrifugal force, which causes the water molecules to move away from the center of the bucket and towards the sides of the bucket. This results in the water forming a concave surface.

Yes, the faster the bucket is spinning, the more pronounced the concave shape of the water will be. This is because the centrifugal force increases with the speed of rotation.

Yes, the water can stay in the bucket even when it is upside down, as long as the bucket is spinning fast enough to create a strong enough centrifugal force to counteract the force of gravity pulling the water downwards.

Similar threads

• Aug 27, 2016
• Jul 23, 2021
• Jun 21, 2023
• Feb 6, 2021
• Apr 9, 2021
• Oct 3, 2018
• Dec 14, 2021
• Aug 19, 2019
• Jun 12, 2016
• Mar 17, 2012

Hot Threads

• I   Is nonlinear acoustics in mainstream physics?
• Insights   How to Solve a Multi-Atwood Machine Assembly
• I   Seemingly a contradiction of conservation of energy?
• B   Why 186,282?
• I   I'm calculating more energy out than I put in

Recent Insights

• Insights   Views On Complex Numbers
• Insights   Addition of Velocities (Velocity Composition) in Special Relativity
• Insights   Schrödinger’s Cat and the Qbit
• Insights   The Slinky Drop Experiment Analysed
• Insights   The Lambert W Function in Finance

Choose an Account to Log In

Notifications

Science project, centripetal and centrifugal force.

Have you ever wondered why you don’t fall out of an upside down loop on a roller coaster, or why a satellite can stay in orbit around the earth? Centripetal force is a force that causes an object to move along a curved path by pulling the object towards the center of the path. The velocity (speed and direction) of the object is constantly changing because the direction of the object is constantly changing, even though the speed remains the same unless acted upon by an outside force. The direction of such an object at any given point is always perpendicular to the centripetal force.

For a circle, the centripetal force is given by the following equation:

F = ma c = ( mv 2 / r )

Where F is the force in Newtons, m is the mass of an object in kilograms, and a c is the centripetal acceleration which can also be described by v 2 /r , the square of the velocity divided by the radius (the distance from the center of the circle).

For objects traveling in a vertical orientation, meaning at some point they are upside down, the centripetal force must be at least equal to the gravitational force, so the object (or person!) does not fall.

Observe centripetal force in action, and use the centripetal force equation to predict the results of the experiment.

Hypothesis:

What will happen to the water in the bucket when the bucket is spun faster? Slower?

• Plastic bucket with handle
• Meter stick
• Notebook and pencil
• Measure your arm from the shoulder to your hand. When spinning the bucket, this will be the radius of the circle. Record the length in meters.
• Weigh the bucket on the scale. Record the weight.
• Place the large jug on the scale and record the weight.
• Pour water into the jug and record the weight. Subtract the weight of the jug to get the weight of the water alone.
• Convert the weight of the water into kilograms. Why is it important that the mass is in kilograms?
• Pour the water into the bucket.
• Go outside to an area where it is okay to spill water. With your arm fully extended, swing the bucket around in circles.
• Swing the bucket slower and slower until the water spills out.
• Using the centripetal force equation, calculate the velocity of your spin for the mass of water in the bucket. How do you solve for velocity? What is significant about this force? This velocity?
• Repeat the experiment with different masses of water, or even different radii by tying a rope to the bucket handle.
• Compare centripetal force to gravity exerted on the water. How much water can you swing for a given velocity?

The water will spill out of the bucket when the gravitational force of the water exceeds the centripetal force exerted on the water when it is upside down.

Centripetal force exerted on a spinning object like our bucket of water also leads to an equal and opposite centrifugal force, the force that the rotating object exerts on the restraining mass (the hand that is swinging the bucket). These two forces work in opposition, which pushes the water to the bottom of the bucket as it passes overhead. Centrifugal force is a consequence of inertia—the tendency of a moving object to want to continue moving in a straight line. As we fling our bucket of water in an arc over our head, the water wants to continue traveling in a straight line, but the force of our hand constantly redirects the water so it travels in an arc instead! Water’s inertia resists this redirection, causing the force of the water to push against the bottom of the bucket. It’s a great example of Newton’s third law: Our hand pulls on the bucket to change the direction of the water from a straight line to an arc (centripetal force), and the centrifugal force from the water pushes the water to the bottom of the bucket!

Here’s an analogous situation: Imagine you’re riding as a passenger in your dad’s car. He makes a really sharp turn, and as a result, you feel like you’re being thrown against the inside of the car door. What’s really happening is that your body wants to continue moving forward, but the turning car pulls your body in a new direction. Your body’s intertia resists this pull, because like all objects, it wants to continue traveling in a straight line.

Now, let’s take a look at the math.

To solve for velocity of your swinging bucket, you have to calculate the gravitational force that acts on the water:

Where F g is the gravitation force in Newtons, m is the mass of the water and g is the acceleration due to gravity, which is 9.81m/s on Earth.

The water will spill from the bucket when the gravitational force is slightly greater than the centripetal (or centrifugal) force, so for simplicity they can be set to equal each other, the variables rearranged, and solved. It is important that weight (mass) is measured in kilograms because that the units in the equation must be consistent for the equation to be true.

Related learning resources

Add to collection, create new collection, new collection, new collection>, sign up to start collecting.

Bookmark this to easily find it later. Then send your curated collection to your children, or put together your own custom lesson plan.

Stack Exchange Network

Stack Exchange network consists of 183 Q&A communities including Stack Overflow , the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Q&A for work

Connect and share knowledge within a single location that is structured and easy to search.

Why does water in spinning bucket move out outward if centripetal and centrifugal forces cancel out?

Evey source I saw keeps saying change in shape of water is due to centrifugal force but it cancels out with centripetal force And if you see from inertial frame of reference the water shouldn't moving just like an object in accelerating bus does not move with respect to inertial frame but moves according to passenger Not taking friction

• newtonian-mechanics
• reference-frames
• inertial-frames
• centripetal-force
• centrifugal-force

• $\begingroup$ More on centripetal & centrifugal forces . $\endgroup$ –  Qmechanic ♦ Commented Feb 12, 2023 at 7:52

3 Answers 3

Copying from here

Centrifugal force (Latin for "center fleeing") describes the tendency of an object following a curved path to fly outwards, away from the center of the curve. It's not really a force; it results from inertia — the tendency of an object to resist any change in its state of rest or motion. Centripetal force is a real force that counteracts the centrifugal force and prevents the object from "flying out," keeping it moving instead with a uniform speed along a circular path.

Italics mine for emphasis.

In the spinning bucket the real force, i.e giving a dp/dt to the liquid, the centripetal, comes from the friction of the walls of the bucket with the molecules of the liquid, kicking them continually into the circular path. The energy is supplied by what is spinning the bucket.

For a full bucket, the spinning bulk liquid has a centripetal force transferred through the bulk by the cohesion of the liquid and the same logic holds: the molecules by inertia would go in straight lines. the centripetal force turns them into circular or vortex path.

Actually Centripetal and Centrifugal forces are the same. It only depends on the frame of refrence of the observer. They can exist simultaneously. Centrifugal force is centripetal force but in the frame of accelerating object rather than being a inward force it becomes a outward force and that is centrifugal force.(You can say that it's a Pseudo Force.).

• $\begingroup$ My question is different read description $\endgroup$ –  Deepak Vss Commented Feb 12, 2023 at 8:35

Your question is based on the assumption that centripetal force and centrifugal force "cancel out." But, they don't cancel out because they are just two different names for the same thing.

When we describe the experiment in terms of an inertial reference frame, we see a person pulling on (exerting a force on) a rope, the rope exerting a force on the bucket, the bucket exerting a force on the water. All of those forces pull the water toward the person, preventing the water from flying away in a straight line.

We call that force, "centripetal."

Issac Newton said that for every action, there is an equal and opposite reaction. What he meant was, We can think of the bucket exerting a force on the water, or we can think of the water exerting an equal and opposite force on the bucket. It amounts to the same thing either way. Really, it's one force acting between the bucket and the water.

In the inertial reference frame, our attention is focused on how the person at the center seems to be pulling the water toward themself. But, when we describe the experiment in terms of a rotating frame—a frame in which the bucket and the person are stationary—we see a different picture. The same force acts between the bucket and the water, but it seems magical because nothing in the picture is moving. The water just seems to mysteriously want to fly away from the person at the coordinate system origin if the person and the rope and the bucket did not hold it back.

When we focus our attention on the force of the water pulling against the bucket, we call that "centrifugal." But, it's the same force pair in either case. So, nothing cancels. A force can't cancel itself.

• $\begingroup$ Maybe my wording is wrong What I am saying is when you shift to inertial frame net force becomes zero like an object in an accelerating truck for someone in truck it appears to move in opposite direction to motion of truck(caused by inertial force) But for someone outside it doesn't move My question why is water moving for someone seeing it from inertial frame $\endgroup$ –  Deepak Vss Commented Feb 13, 2023 at 4:56
• $\begingroup$ @DeepakVss, Why the bucket moves in an inertial frame is also why the bucket moves in most frames. It's because the bucket is moving in reality. Why the bucket does not move in one particular rotating frame is because we carefully chose that frame such that the coordinates of the bucket do not change with time. (I.e., it does not move _relative to that special frame.) $\endgroup$ –  Solomon Slow Commented Feb 13, 2023 at 12:51
• $\begingroup$ P.S., What's special about inertial frames is, in any inertial frame, we can explain that the force between the bucket and the person at the center is due to the person pulling against the inertia of the bucket. There also is a simple law to explain the force in the one, carefully chosen, rotating frame where the bucket does not move: Everything in that frame is pulled away from the origin by "centrifugal force." But that law does not generalize to other frames, and so it does not help us to understand the reality. That's why we call the mysterious centrifugal force a "pseudo force." $\endgroup$ –  Solomon Slow Commented Feb 13, 2023 at 12:55

Your Answer

Sign up or log in, post as a guest.

Required, but never shown

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy .

Not the answer you're looking for? Browse other questions tagged newtonian-mechanics reference-frames inertial-frames centripetal-force centrifugal-force or ask your own question .

• Featured on Meta
• Upcoming sign-up experiments related to tags

Hot Network Questions

• How to re-use QGIS map layout for other projects /How to create a print layout template?
• Is a judge's completely arbitrary determination of credibilty subject to appeal?
• RMS of sine wave curve defined between two points
• What camera could I mount or rig to my bike that can read/recognize license plates
• How to Transform a PSBall into Other Shapes
• Will it break Counterspell if the trigger becomes "perceive" instead of "see"?
• Are there good examples of regular life being theory-laden?
• Best practices for relicensing what was once a derivative work
• Which computer first used stored characters (shape selector) and character bit patterns in the same memory
• Can I be denied to board a cruise at a US port if I have a misdemeanor?
• Short story crashing landing on a planet and taking shelter in a building that had automated defenses
• Arrays. Find row with most 1's, in O(n)
• What was the Nuclear Boy Scout's Eagle Scout Project?
• Why is my bike clunky, and what is this loose string?
• Is there a second-order non-linear addition to Maxwell's equations?
• How can you destroy a mage hand?
• Patentability of graphs
• Movie with a gate guarded by two statues
• How to make a low-poly version of a ramen bowl?
• How to send transaction bundles?
• Reviewers do not live up to own quality standards
• Why was the SNES color gamut RGB?
• 3 doors guarded by 1 knight and 2 knaves
• Is it better to perform multiple paired t-test or One-Way ANOVA test