# Homework Help: Need help with this cons. of momentum prob.

1. Oct 20, 2004

### nazarip

When a bullet strikes a block of wood originally at rest, the bullet becomes embedded into the wood block which travels 1.65 m after the collision. If the mass of the bullet is 4.17 g and the mass of the block is 2.00 kg, what is the speed of the block immediately after the collision? The coefficient of kinetic friction between the block and the surface is 0.10.

I know i need to find the initial velocity of the bullet but i am not sure how...

2. Oct 20, 2004

### Sirus

The trick here is factoring in the frictional forces. Have you tried anything so far?

3. Oct 20, 2004

### nazarip

I did try using newtons second law to determine the net force on the wood block. In this case I would have to factor in the frictional force...ie.
F(bullet)-F(friction)=m(bullet+block)a, but i ran into a wall because i dont have the acceleration or F(bullet).

4. Oct 20, 2004

### Sirus

Are you sure that is the net force acting on the block and bullet during their travel over the surface?

5. Oct 20, 2004

### nazarip

Well, there is the force of gravity and the normal force but they are in the vertical direction...all that tells me is that the normal force equals the weight and i can get the frictional force with that piece of info...i dont believe there are any other forces acting on the wood block, 2 in the vertical direction and 2 in the horizontal direction, but I could be (and probobly am) wrong.

6. Oct 20, 2004

### Sirus

You said net force is $F_{bullet}-F_{friction}$. That is incorrect. Can you figure out why?

7. Oct 20, 2004

### nazarip

Hmm. Well, is it just -F(friction)?Since the bullet is embeded in the block it has zero acceleration and thus zero F...that might work...

8. Oct 20, 2004

### nazarip

Just did some number crunching, and it worked...thanks for the patience.

9. Oct 21, 2004

### Sirus

You are welcome.