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## Homework Statement

A ballistic pendulum is a device that may be used to measure the muzzle speed of a bullet. It is composed of a wooden block suspended from a horizontal support by cords attached at each end. A bullet is shot into the block, and as a result of the perfectly inelastic impact, the block swings upward. Consider a bullet (mass

*m*) with velocity

*v*as it enters the block (mass

*M*). The length of the cords supporting the block each have length L. The maximum height to which the block swings upward after impact is denoted by

*y*, and the maximum horizontal displacement is denoted by

*x*.

a) In terms of

*m, M, g and y,*determine the speed of the bullet.

## Homework Equations

mgh=1/2 m v

^{2}

## The Attempt at a Solution

(1/2)(

*m*)v

^{2}= (

*m + M*)(g)(

*y*)

v =sqroot { [(2)(g)(m + M)(y)]/m }

The real solution states that one must find the initial velocity of the bullet-block system, and then use this equation. Why can I not just use the initial velocity of the bullet? The real final answer is what I have, except the (m+M)/m is outside the sqroot.