What is the rest mass of a body if it explodes into two identical objects of rest mass 3 kg each, moving in opposite directions with a speed of 0.7c relative to the original body?

The proper mean lifetime of π+ mesons travelling with a velocity of 0.8c is 2.4 × 10-8 seconds. Its apparent mean lifetime as observed from the lab frame is:

If L03 is the rest volume of a cube, then the volume viewed from a reference frame moving with uniform velocity v in a direction parallel to an edge of the cube is: