Question

What happens in terms of energy when a moving car hits a parked car, causing the parked car to move?

The moving car transfers kinetic energy to the parked car.
Kinetic energy in the moving car disappears.
Kinetic energy in the parked car is created.
The parked car transfers kinetic energy to the moving car.

Answer 1

Correct answer: only the first statement is essentially true.

Explanation:
- The moving car does transfer kinetic energy to the parked car (via the contact forces during the collision), so the parked car gains kinetic energy.
- Kinetic energy of the moving car does not "disappear" — some of it is lost from that car but energy is conserved: that lost energy is converted into other forms (kinetic energy of the other car, heat, sound, permanent deformation of the cars, etc.).
- Kinetic energy in the parked car is not magically created; it comes from the moving car (and from the energy conversions that occur during the impact).
- The parked car does not (in this typical scenario) transfer kinetic energy to the moving car. During the collision both bodies exert forces on each other and there can be temporary exchange, but the net transfer is from the initially moving car to the previously parked one.

(Also note: momentum of the two-car system is conserved in the collision if external impulses are negligible; how the kinetic energy is shared depends on how elastic the collision is.)