# Why do bullets ricochet?

## How does a spherical pendulum work?

You have probably seen a ball pendulum like this before: Several small metal balls are hung one behind the other on a rod with thin threads. If you let the front ball hit the row of balls, the ball at the rear end of the row moves away - without the balls moving in between. Is that magic? No! This is the physical principle of conservation of momentum.

The word Conservation of momentum is derived from the Latin word "impellere", which means "to touch, to set in motion". And that's exactly what happens when you let the outer ball slip against the next one. You kick the next bullet. In a physically correct form, a collision is the collision of two bodies moving against each other (in this case from ball against ball). This creates an impulse: a body is set in motion in a certain direction. This can be seen well in billiards, for example - if you hit the white ball with the cue and if it hits another ball, it will start moving in a certain direction.

The following happens with the ball pendulum: The outer ball hits the next one. This cannot swing, however, because there is another ball next to it in the direction of swing. Therefore, the one hit by the outer ball simply transfers the impulse to the next in the direction of the swing, which then transfers the impulse to the next one. This continues until the impulse has reached the point where it can execute the movement: The outer ball swings away! The momentum is passed on from ball to ball and remains the same if there are no friction losses such as air resistance. It is important that the balls are made of the same material and of the same size.

If the colliding materials are not solid but deformable, something else happens: The kinetic energy that ensures that the momentum is maintained with the same force in the spherical pendulum is converted. If two deformable bodies collide, the momentum is converted into energy, which deforms the two bodies. This is how it would be with a ball pendulum made of plasticine. If the outer ball touches the next, the putty is deformed - then there is nothing to swing.

This phenomenon is also used, e.g. in the crumple zone of cars. These are intentionally built in such a way that the momentum of an impact is not retained in the event of a collision, but is diverted as energy into the deformation of the crumple zone. That protects the inmates. You will not be shaken that much.
Ah!