Question

a bullet of mass 7 g is fired into a block of metal weighing 7 kg. the block is free to move. after the impact , the velocity of the bullet and the block is 70 m/s. what is the initial velocity of bullet?

Answer 1

If the bullet of mass m1 is fired with velocity u1 into a block of mass m2 which is at rest and the combine velocity of bullet-block system is v then,

By law of conservation of momentum
m1u1 = (m1 + m2)v

u1 = (m1 + m2)v / m1
= (0.007 + 7) * 70 / 0.007 ………[∵ 7g = 0.007 kg]
= 70070 m/s