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.
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?