A 1200.-kg car runs into the back of a 2000.-kg car stopped at a red light. From

Question

A 1200.-kg car runs into the back of a 2000.-kg car stopped at a red light. From the fact that the joined vehicles skidded forward 4.00 m, the investigating police officer is able to conclude that the speed after the collision was 6.60 m/s.

What happens to the kinetic energy of the system in this kind of collision?

The kinetic energy is conserved.

The kinetic energy is reduced to 0.

The kinetic energy is reduced (but not to 0).

The kinetic energy increases (slightly).

The kinetic energy is conserved.

The kinetic energy is reduced to 0.

The kinetic energy is reduced (but not to 0).

The kinetic energy increases (slightly).

Explanation / Answer

During a collision the objects involved generally apply equal-and-opposite forces on one another for a short time. There are usually no external forces, so the momentum of the system of objects is conserved.

Generally, momentum is conserved in all types of collisions.

There are four classes of collisions based on what happens during the collision and, in particular, what happens to the total kinetic energy of the system.

The elasticity of the collision is related to the ratio of the relative velocities of the two colliding objects after and before the collision

In this case, this collision is inelastic as the two vehicles are joined, so the correct option is the third one.

Hope this helps

Type of Collision Description Elasticity Super-elastic Kinetic energy is larger after the collision (e.g., an explosion) k > 1 Elastic Kinetic energy is conserved k = 1 Inelastic Kinetic energy is smaller after the collision k < 1 Completely inelastic Kinetic energy is smaller, and the objects stick together, after the collision. k = 0

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