# Homework Help: Lab help Ballistic pendulum

1. May 2, 2004

### ussjt

A steel marble is shot out of a launcher (straight) in a "catcher" on a ballistic pendulum. The pendulum then swings up into its max height ans stops. At that point...the height for the gPE is .163 m. The mass of the marble is .0558g. I need to find the inital velocity of the marble. I solve (.5)(m)(v^2)=mgh to find the starting velocity for that, which is the final velocity after the perfctly inelasic collusion. But what happens is that I use the perfctly inelasic collusion formula, m1v1 + m2v2 = (m1 +m2)v' , but I always end up with two variable because I don't know m2. This is where I get stuck. I know the velocity should be some where around 3.5, but I'm not getting close. Please help me..I attached a diagram, I hope it works.

2. May 2, 2004

### ussjt

Sorry for the double post...but could someone one help me ASAP. Thanks.

3. May 2, 2004

### arildno

Can't find the diagram, but:
What do you get if you assume the mass of the pendulum to be much less (negligible) to the mass of the steel ball?

4. May 2, 2004

### Staff: Mentor

Right, assuming you really mean: (.5)(m1 + m2)(v'^2)=mgh.
Since the speed of the "catcher" is zero before the collision, you mean:
m1v1 = (m1 +m2)v', where v1 is the speed of the marble.

There's no way around it: You need the mass of the "catcher" if you wish to find v1.