### Two carts

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

How does it work
1. Despite the fact that only one person pulls the rope both carts move. This is due to the Newton’s third law of motion: applying the force we experience the work of force of the same value but opposite direction. The point of collision depends on each cart’s momentum. In the configuration of the two carts, the momentum is conserved, which means that it stays the same before and after the collision. Since the momentum is the product of mass and velocity, increasing the mass on the second cart results in the velocity decreasing, and as a consequence in change of the collision point.
2. During the collision, the immobile cart takes over the energy of the moving cart (the energy is conserved as well). As a consequence of the energy conveyance, the cart pulled by us stops. If the mass of the carts is not equal both carts move after the collision. Such a situation only takes place in case of elastic collision. If the carts stuck together the collision would be inelastic.

Interesting facts
The momentum conservation is the principle of operation of jet engines, where the momentum of released gases is equal to the momentum of the vehicle (but oppositely directed).