Two particles of masses m1 and m2 in projectile motion have velocities →v1 and →v2 respectively at time t = 0 and that is the moment when they collide. Their velocities become →v′1 and →v′2 at time 2t0 while still moving in air. The value of magnitude of change in total momentum would be
2(m1+m2)gt0
The momentum of the two-particle system, at t = 0 is
→pi=m1→v1+m2→v2
Collision between the two does not affect the total momentum of the system.
A constant external force (m1+m2)g acts on the system.
The impulse given by this force, in time t = 0 to t=2t0 is (m1+m2)g×2t0
∴ Change in momentum in this interval =|m1→v1+m2→v2−(m1→v1+m2→v2)|=2(m1+m2)gt0