A ballistics pendulum is a device used to measure the speed of a fast moving projectile such as a bullet. A bullet of mass 5g is fired into a large block of wood of mass 1.5kg suspended from some light wires. The bullet is stopped by the block and the entire system swings through a vertical distance of 5cm. Find the initial speed of the bullet.

Question

A ballistics pendulum is a device used to measure the speed of a fast moving projectile such as a bullet. A bullet of mass 5g is fired into a large block of wood of mass 1.5kg suspended from some light wires. The bullet is stopped by the block and the entire system swings through a vertical distance of 5cm. Find the initial speed of the bullet. 
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Damon · Accepted Answer

initial momentum = .005 v
momentum after impact = .005 v = 1.5 V
so
V = .005 v/1.5 = .00333 v

kinetic energy after impact = (1/2)(1.5)V^2 = 8.32*10^-6 v^2

final potential energy = mgh=1.5*9.81*.05
= 8.32*10^-6 v^2
so
v^2 = 88467
v=297 m/s