You know that a collision must be "elastic" if: a. The colliding objects stick together b. The colliding objects are stretchy or squishy c. The sum of the final kinetic energies equals the sum of the initial kinetic energies d. There is no change in the internal energies of the objects (thermal energy, vibrational energy, etc.) e. The momentum of the two-object system doesn't changeWhich one is true/false?

In a crash the total momentum of the system must be preserved, so when defining the system all bodies must be included, when the bodies collide and bounce between them a redistribution of the moment between the bodies occurs.

The elastic shock is defined when the initial and final kinetic energy do not change and the inelastic shock when the kinetic energy changes.

Now let's apply these concepts to the statements given

a) False. If the bodies join some energy it is used for the union and therefore the kinetic energy changes

b) False. In general, soft bodies tend to stick during the crash losing kinetic energy

c) true. This is the definition of elastic shock

d) true Even when the elastic shock does not depend on internal energy, if it does not change all the kinetic energy must be conserved

e) False. The impulse is redistributed between the bodies, so it changes