Carts on rails

What you can do here
  1. Put an empty cart in the middle. Push the other empty cart towards it, so that they collide. What happens to the carts? Will the result of the experiment be different if there is a person sitting on the colliding cart.
  2. Place the carts at the end of the tracks. Sit on one cart and pull the other, empty cart using a rope. Notice the exact collision point. Repeat the experiment with another person sitting in the other cart. Has the collision point changed?


How it works
  1. The colliding carts demonstrate elastic the collision where not only the momentum, but also, kinetic energy is conserved. If the carts have equal mass, the pulled cart stops as a consequence of momentum and kinetic energy conveyance. If they don’t have equal mass, both carts move after the collision. If the carts stuck together, the collision would be perfectly inelastic with the momentum conserved but kinetic energy partially dissipated.
  2. Despite the fact that only one person pulls the rope both carts move. This is because of Newton’s third law of motion: if you apply force, you experience the force of the same magnitude but in the opposite direction. Frequently this is referred to as the law of action and reaction. The collision point depends on the velocity of each cart. Equal a force acts on the carts for the same amount of time so they gain momentum of the same value. Since the momentum is the product of mass and velocity, increasing the mass on the second cart results in a decrease in its velocity, and consequently in the change of the collision point.


Interesting facts

Momentum conservation is the principle of operation of jet engines where the momentum of the released gases is equal to the momentum of the vehicle (but in the opposite direction).