Homework Help: Ballistic Pendulum Rifle

1. Nov 4, 2008

Tohte

The problem:

https://tycho-s.phys.washington.edu/cgi/courses/shell/common/showme.pl?courses/phys121/autumn08/homework/06/IE_algebra_ballistic_pendulum_MOM/pic.gif [Broken]

A rifle bullet of mass m = 0.03 kg traveling at vb = 240 m/s collides with and embeds itself in a pendulum of mass M = 2.88 kg, initially at rest and suspended vertically by massless strings of length L = 2 m.

How high does the pendulum-bullet combination rise after the collision?

I don't know the process for solving this equation, so help would be appreciated.

Last edited by a moderator: May 3, 2017
2. Nov 4, 2008

Staff: Mentor

Energy preservation. Kinetic gets converted into potential.

3. Nov 4, 2008

Tohte

Okay, so I need to use PE = mgh. I don't understand what I do with the velocity of the bullet and how to setup the problem, if I could get a little more guidance it would help me out

Thanks.

4. Nov 4, 2008

Staff: Mentor

What is formula for kinetic energy?

5. Nov 4, 2008

shutoutsteve

We did this the other day, except we just manipulated the formulas instead of working with actual values.
I am going to attempt to make sense of what I have for that note, so you may hear from me soon...

6. Nov 4, 2008

Tohte

KE = (1/2)mv2

7. Nov 4, 2008

Staff: Mentor

Is there something you still don't know? Just beware which mass is which - ballistic pendulum mass after hit is that of pendulum PLUS bullet.

8. Nov 4, 2008

Tohte

I don't understand what it is I do with the equation for Kinetic Energy, I'm supposed to find the height of the bullet-pendulum system after the bullet hits the pendulum.

9. Nov 4, 2008

Staff: Mentor

h as of mgh.

10. Nov 4, 2008

Staff: Mentor

This problem is best solved in two stages:
(1) The collision itself. You need to find the speed of the block+bullet immediately after the collision. Hint: Energy is not conserved, but something else is.
(2) The rising of the pendulum after the collision. Here you do use energy conservation, as Borek explains.

11. Nov 4, 2008

Staff: Mentor

Do we have an emoticon for selffishslapping?