Carts on tracks

HOW

1. Place an empty cart in the middle of the track.
2. Push the other empty cart towards it so that there is a collision. How do the carts behave
after the crash?
3. Check whether the result of the experiment changes if someone sits on the impact cart.
4. Position the carts at the ends of the track. Sit on one of them and, using a rope, pull the other, the empty one. Note where the two carts collide.
5. Repeat the experiment with a second person in the second cart. Does the crash point change?

WHY

The cart collision is an example of an elastic collision in which not only momentum but also kinetic energy is conserved.

• If the masses of the carts are equal, the consequence of the transmission of momentum and kinetic energy is that the cart you are pushing stops.

• If the mass of the carts is not equal, both carts will move after the collision.

• If the carts are “stuck” together, it would be a perfectly inelastic collision, in which momentum is conserved but some kinetic energy is lost.

Despite only one person pulling the rope, both carts move. This is a manifestation of Newton’s third law
of motion: by acting on the other cart with a certain force, you also experience a force of the same value,
but the opposite direction. This principle is often referred to as the principle of action equals reaction.

The point of collision depends on the speed of both carts. A force of the same magnitude and for the same duration acts on both carts. Therefore, they obtain momentum of the same value. Since momentum is the product of mass and velocity, the loading (increase in mass) of the second cart results in a decrease in its velocity and, consequently, a change in the collision point.