# Homework Help: Stuck on a momentum question

1. Oct 9, 2006

a 10g bullet is fired into a 10kg wood block that is at rest on a wood table. The block, with the bullet embedded, slides 5.0cm across the table. What was the speed of the bullet?

So my question is how do I get started, this problem has had me stuck for awhile. Thanks

2. Oct 9, 2006

### Chi Meson

Work backwards. First assume that the bullet+block lose all kinetic energy to friction. How much kinetic energy would it have had ?

3. Oct 9, 2006

### OlderDan

The problem does not contain all the information you need. Perhaps you are expected to look something up. The missing ingredient has to do with the friction between the block and the table.

4. Oct 9, 2006

ok, the friction between wood and wood says its .2

5. Oct 9, 2006

### OlderDan

Use this information to determine the frictional force acting on the block as it slides the 5cm. From the force you can find the acceleration. From the acceleration and distance you can find the initial velocity of the block just after impact. The momentum of the block with imbedded bullet just after impact has to be the same as the bullet before impact (block initially at rest). The actual process is a bit more complicated because the block starts sliding before the bullet stops penetrating, but you can assume the bullet and block have the same velocity as soon as they touch.

6. Oct 9, 2006