Two cars with the same masses m are moving at speeds v and 2v relative to the Earth in the same direction.

Two cars with the same masses m are moving at speeds v and 2v relative to the Earth in the same direction. What is the kinetic energy of the second car in the frame of reference associated with the first car?

If the cars move in one direction, then the speed of the second car relative to the first is v, the kinetic energy is:
E = mv ^ 2/2
If the cars move in different directions, then the speed of the second car relative to the first is 3v, the kinetic
energy is equal to:
E = m * (3v) ^ 2/2 = 4.5mv2