In the image below, if the engine backs up in order to couple (join) with several more train cars and push them backwards, what explanation best describes the type of collision it is?

A. It is an inelastic collision because the collision conserves momentum.

B. It is an inelastic collision because the train cars stick together and move as one.

C. It is an elastic collision because the collision conserves momentum.

D. It is an elastic collision because the cars stick together and move as one unit.

B. It is an inelastic collision because the train cars stick together and move as one.

Explanation:

Momentum

When two or more objects collide in a closed system (no external forces are acting) the total momentum is conserved:

where m1...m2 are the masses of each object, v1...vn are their velocities before the collision takes place and v'1...v'n are their velocities after the collision.

If a collision is elastic, then the kinetic energy is also conserved. when the collision is inelastic, part of the initial kinetic energy is lost. A typical case of inelastic collision occurs when the objects join and remain together after the collision. The velocity is common to all of them and the mass is the sum of the individual masses.

This is exactly the case described in the question: serveral train cars are joined and continue moving together after the collision. It corresponds to a inelastic collision described in the option B.

total distance/total time

(85 + 82)/(5+3)

167/8

20 just over