To an observer, two bodies of equal rest mass collide head on with equal but opposite velocities 4c/5 and cohere. To a second observer, one body is initially at rest. Find the apparent velocity of the other, moving mass before the collision and compare its initial energy in the two frames of reference.

To an observer, two bodies of equal rest mass collide head on with equal but opposite velocities 4c/5 and cohere. To a second observer, one body is initially at rest. Find the apparent velocity of the other, moving mass before the collision and compare its initial energy in the two frames of reference.