Question
A bullet of mass 20 g is horizontally fired with a horizontal velocity 150 ms–1 from a pistol of mass 2 kg. What is the recoil velocity of the pistol?
Solution
For bullet:
m1 = 20 g = 0.02kg
u1 = 0 m/s
v1 = 150 ms-1
For pistol:
m1 = 2kg
u1 = 0 m/s
v2 = ?
According to the law of conservation of momentum,
Total momenta after the fire = Total momenta before the fire.
That is,
m1v1 + m2v2 = m1u1 + m2u2
0.20 x 150 + 2 x v2 = 0.02 x 0 + 2 x 0
2v2 = -3
v2 = -1.5 ms-1.
Negative sign indicates that the direction in which the pistol would recoil is opposite to that of bullet.
