A Newton's cradle is a classic physics project that shows how motion can pass through a row of touching objects. In this cardboard version, five identical metal nuts or beads hang from fishing line inside a simple frame. When one ball is pulled back and released, it hits the row and usually one ball on the far side swings out.
This makes the project a fun way to see conservation of momentum and energy in action.
Understanding Make a Cardboard Newton's Cradle
The important part of a cradle is not just the swinging ball. It is the brief squeeze that happens when the balls collide. Metal balls deform by a tiny amount at contact.
That deformation stores energy for a moment, much like a very stiff spring. The ball at the far end receives a push as the squeezed row pushes back. Because the middle balls are trapped between neighbors, they move very little as a group.
Their job is to carry the contact force across the line. This is why a clean, straight row produces the familiar result.
The strings need careful setup. Each hanging ball should be supported by two equal lengths of fishing line, one on each side of the frame. This keeps each ball moving in one flat plane instead of twisting or circling.
The support points should be directly above the line of balls. If one ball hangs lower, it may strike too early or too late.
If there is even a small gap between balls, the collisions happen separately and the motion looks messy. Adjust the knots slowly until every ball just touches its neighbors while hanging still.
A useful test is to release one ball, then two balls held together, from the same height. With a well-built cradle, one ball should leave on the opposite side after the first test. In the second test, two balls should leave together.
This pattern gives evidence that both momentum and motion energy are being shared in the collision. The result is only approximate because real materials are not perfect.
Softer beads, loose knots, bent paperclips, or a flexible cardboard frame absorb more energy. A sharp click is evidence that some energy has become sound.
Students meet the same ideas in many places. A moving shopping cart transfers motion when it bumps another cart. A cue ball sends motion through pool balls.
Car safety tests use collision ideas to study how forces change motion over a short time. This project shows why engineers care about material choice and alignment. When recording results, watch the release height, the number of balls released, how far the outgoing balls rise, and how many swings occur before stopping.
Repeat each trial several times. Small differences are normal, and they help show how measurement and careful construction affect an experiment.
Key Facts
- Momentum is mass times velocity: p = mv.
- Kinetic energy is energy of motion: KE = 1/2 mv^2.
- In a collision, total momentum before the collision is about equal to total momentum after the collision.
- A Newton's cradle works best when all 5 balls have the same mass and touch in a straight line.
- Pulling a ball higher gives it more gravitational potential energy: PE = mgh.
- Some energy is lost to sound, heat, air resistance, and bending of the strings, so the cradle eventually stops.
Vocabulary
- Momentum
- Momentum is a measure of how hard it is to stop a moving object and depends on its mass and velocity.
- Kinetic Energy
- Kinetic energy is the energy an object has because it is moving.
- Potential Energy
- Potential energy is stored energy, such as the energy a lifted ball has because of its height.
- Collision
- A collision happens when two or more objects bump into each other and push on each other for a short time.
- Conservation
- Conservation means that the total amount of something, such as momentum or energy, stays the same in a closed system.
Common Mistakes to Avoid
- Using balls with different masses: this is wrong because a Newton's cradle works best when each hanging object has the same mass, so momentum transfers evenly.
- Leaving gaps between the middle balls: this is wrong because the balls should touch in a straight row for the push to transfer clearly through the chain.
- Tying strings at different lengths: this is wrong because uneven string lengths make the balls swing on different paths and collide off center.
- Making the cardboard frame too weak: this is wrong because a wobbly frame absorbs energy and changes the motion of the cradle.
Practice Questions
- 1 A metal nut has a mass of 0.020 kg and swings at 1.5 m/s just before hitting the row. What is its momentum? Use p = mv.
- 2 A 0.030 kg bead is lifted 0.10 m above its lowest point. How much gravitational potential energy does it gain? Use PE = mgh with g = 9.8 m/s^2.
- 3 If you pull back two balls on the left side of a 5-ball Newton's cradle and release them, why do about two balls swing out on the right side instead of only one?