A relativistic particle with rest mass m collides with a stationary particle of mass M and activates a reaction leading to formation of new particles: m + M → m1 + m2 + . . ., where the rest masses of newly formed particles are written on the right-hand side. Making use of the invariance of the quantity E2 – p2c2, demonstrate that the threshold kinetic energy of the particle m required for this reaction is defined by Eq. (6.7c).