# Physics : Work Energy Power Question

1. Jun 20, 2007

### Gunman

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

A bullet of mass 0.0035 kg is shot into a wooden block of mass 0.121 kg.

They rise to a final height of 0.547 m as shown. What was the initial speed (in m/s) of the bullet before it hit the block?

http://www.physics247.com/physics-homework-help/conservation_nrg_quiz1.php" [Broken]

There is a picture there on how the setting looks like.

2. Relevant equations

3. The attempt at a solution

K.E of bullet = P.E of the total mass at its final height
1/2mv(square) = (m+M)gh
(1/2)(0.0035)v(square) = (0.0035 + 0.121)(9.81)(0.547)
v = 19.5m/s

The actual answer is 116.5m/s.

Where did I go wrong?

Ya. Thanks for any help provided. And since this is my first time posting forgive me if there are any mistakes in the way I posted the question. Thankx again. =)

Last edited by a moderator: May 2, 2017
2. Jun 20, 2007

### Staff: Mentor

Here's where you went wrong: Mechanical energy is not conserved during the collision (some of the bullet's kinetic energy becomes thermal and deformation energy). But another physical quantity is conserved during any collision--what is that?

After the collision, the mechanical energy of "bullet + block" is conserved.

3. Jun 20, 2007

### Gunman

Thanks man. =) I got it. So the momemtum of the system is conserved. So by using that I can solve this. Hm..Thank you very much for clearing my misconception. :)

4. Mar 24, 2011