[A Level] Logic behind the graph?

Ryan is examining the energy of the particles in a bar of gold. what is ryan most likely studying?

The force it would take to accelerate an 900-kg car at the rate of 6m/s2

Arailroad car collides with and sticks to an identical railroad car that is initially at rest. after the collision, the total kinetic energy of the two cars is a) the same as before. b) half as much as before. c) one third as much as before. d) one fourth as much as before. e) twice as much as before.