SfC Home > Physical Science >

Explanation of Newton's Cradle - Succeed in Physical Science. Also refer to physics, Isaac Newton, Simon Prebble, Law of Conservation of Momentum, mass, velocity, energy, friction, inertia, pendulum, Ron Kurtus, School for Champions. Copyright © Restrictions

Newton's Cradle

by Ron Kurtus (revised 12 November 2006)

Newton's Cradle is a clever device that not only demonstrates a pendulum, but also shows the Laws of Conservation of Momentum and Energy. Friction and damping effects are also observed. This device is often used as a toy to amuse people. Newton's Cradle was invented in 1967 by English actor Simon Prebble and named in honor of scientist and mathematician Isaac Newton because it employs Newton's Laws.

Questions you may have include:

• What is Newton's Cradle?
• What does it look like?
• Why does it work?

This lesson will answer those questions. There is a mini-quiz near the end of the lesson.

Useful tools: Metric-English Conversion | Scientific Calculator.

Note: If you want to hear the text being read, click the Play button. It takes a few seconds for the sound to start. The voices are somewhat mechanical for computer use.

Time = 4 min. 14 sec.

Newton's Cradle

Newton's Cradle consists of several metal balls (usually 5) suspended from a rack by wires or rods, such that they line up and are in contact when in a resting position. There are two wires attached to each ball to keep the pendulum motion in one plane.

Pull up one ball

When an end ball (ball #1) is pulled up and let go, it swings down as a pendulum and hits the next ball. The energy and momentum from that ball is transmitted through the three balls at rest to the ball on the other end (ball #5). That ball is propelled forward at the same speed as the first ball had due to the force of the first collision.

This process continues at ball #5 reaches its peak and then swings down to hit the balls at rest, propelling ball #1 forward and upward.

One ball raised and ready to swing

Two or more balls

If two or more balls are pulled up and let go, the same number of balls will be propelled forward on the other end, due to the collision.

Slowly slows down

Since it is a form of pendulum, the action will go back and forth until it slowly slows down due to losses from friction and the elasticity of the balls.

Simulation

The following simulation allows you to explore Newton's Cradle on your computer similar to using the real device. (Note that you must have the Flash player installed in your computer to use this simulation.)

Drag on one ball

Place you mouse pointer on an end ball, hold down the left mouse button, and drag the ball, so it is at an angle. Then release the mouse button and let the ball swing free. You will see that only one ball on the other end of the group swings up at about the same speed as the ball you let go.

(Flash animation designed by Bryan Heisey)

Notice that the balls start to slow down and will bounce less and less until they finally stop. This is due to losses from friction and energy that is absorbed in the balls. The effect is called damping of the periodic motion.

Drag multiple balls

You can drag two, three or four balls and let them go. The same number of balls you release will be moved forward upon the collision with the moving balls. This verifies the Law of the Conservation of Momentum, which states that the momentum (mass times velocity) remains the same after a collision.

Explanation and equations

When one or more balls are pulled up and let go, they swing down as a pendulum. The velocity (v) of the balls, when they reach the hit the resting balls is determined by the height (h) from where they were let go and the acceleration of gravity (g). The velocity is approximately v = SQRT(2h/g).

The kinetic energy of the balls is KE = mv²/2 and the momentum is p = mv, where m is the mass of the balls in motion.

Because the momentum and energy must be maintained in this system, the balls that move will have the same velocity and total mass as the balls that were in initial motion. Thus, if two balls are let go, two balls will be sent in motion after the collision.

Of course, friction and losses due to elasticity will slowly reduce the speed of the balls.

Summary

Newton's Cradle demonstrates laws of motion, including the Laws of Conservation of Momentum and Energy. The simulation allows you to experiment swinging different number of balls to verify the conservation of momentum.

Answers to Readers' Questions

Once you work toward a goal, your inertia moves you toward success

Resources

The following resources provide information on this subject:

Websites

Conservation of Momentum - Mathematical explanation from the University of Winnipeg, Canada

Physical Science Resources

Books

Top-rated books on Laws of Motion

Miscellaneous

Mini-quiz to check your understanding

1. What laws are demonstrated with Newton's Cradle?

2. What would happen if each ball was held by only one wire?

3. Why won't the balls bounce forever?

If you got all three correct, you are on your way to becoming a Champion in Physical Science. If you had problems, you had better look over the material again.

What do you think?

Do you have any questions, comments, or opinions on this subject? If so, send an email with your feedback. We will try to get back to you as soon as possible.

Share link

Feel free to establish a link from your website to pages in this site.

Or use our form to send this link to yourself or a friend.

Students and researchers

The Web address of this page is
www.school-for-champions.com/science/newtons_cradle.htm.

Please include it as a reference in your report, document, or thesis.

Where can you go from here?

School for Champions

Physical Science topics

Newton's Cradle