# Inelastic/elastic collision?

1. Aug 7, 2009

### ariana0923

The problem statement, all variables and given/known data

A bullet with a mass of 6.00 g is fired through a 1.25 kg block of wood on a frictionless surface. The initial speed of the bullet is 896 m/s, and the speed of the bullet after it exits the block is 435 m/s. At what speed does the block move after the bullet passes through it?

The attempt at a solution

Should I treat this as an inelastic collision even though the two items separate at the end?

If so, using the (m1)(v1)i + (m2)(v2)i = (m1)(v1)f + (m2)(v2)f formula, I got v2f (final vel. of block) = 2.21 m/s

Do you agree?

2. Aug 7, 2009

### Staff: Mentor

Looks good.

The collision is inelastic, because KE is not conserved. (Calculate it and see for yourself.) It's not a completely inelastic collision, because they do separate. In any case, all you need to solve it is momentum conservation.

3. Aug 8, 2009

### Fightfish

Most collisions result in the two bodies separating in the end; the case whereby the two bodies coalesce together is a special case of inelastic collisions - a perfectly inelastic collision.