Question

Object A has of mass 7.20 kilograms, and object B has a mass of 5.75 kilograms. The two objects move along a straight line toward each other with velocities +2.00 meters/second and -1.30 meters/second respectively. What is the total kinetic energy of the objects after the collision, if the collision is perfectly elastic?
19.3 joules
21.9 joules
38.5 joules
43.0 joules
50.8 joules

Answer 1

If the collision is elastic, there is no loss in kinetic energies, which means that the total energies before and after impact are the same.  So no need to worry about final velocities.

Final energy
= initial energy
= (1/2) (7.20*2.00^2+5.75*(-1.30)^2)
=19.26 joules

Answer: the total kinetic energy is 19.3 J. after collision.