Ballistic Pendulum problem

1. Apr 7, 2009

scheng12

1. The problem statement, all variables and given/known data

During a summer internship with a company, you devised the following method of measuring the muzzle speed of a high-powered rifle, as shown in the figure. You fire a bullet into a rod of mass 5.5 kg and length 150 cm that is free to rotate about the pivot at the top. The bullet, whose mass is 8.55 g, strikes at the center of mass of the rod and remains embedded. You measure the maximum opening angle to be 11.5 degrees. What is the muzzle speed in m/s?

2. Relevant equations
I know the equation to solve for the muzzle speed for a normal ballistic pendulum is

v0=(M+m)$$\sqrt{}2gL(1-cos\vartheta)$$/m

3. The attempt at a solution
When I plug the values in the answer comes out to be false since the bullet hits the rod in the center, instead of at the tip. I also tried to plug in L/2 but that doesn't work either. Any ideas?

2. Apr 8, 2009

any1?

3. Apr 8, 2009

omegacore

Instead of just applying the formula you might want to consider where the formula comes from.

A simple energy relationship will tell you that your square root is in the wrong place.

L = L(overall)/2 this is correct.

4. Apr 8, 2009

BMWPower06

where is the square root supposed to be?

5. Apr 9, 2009

omegacore

The square root should be over everything on the right hand side.