# Completely inelastic collision

1. Oct 6, 2007

### ehrenfest

1. The problem statement, all variables and given/known data
Is it true that a completely inelastic collision can be elastic (i.e. KE could be conserved) such as when you fire a gun at a metal ball at rest and the bullet and the metal ball stick togethor? Isn't completely inelastic somewhat of a misnomer since it implies that the collision is not elastic?

2. Relevant equations

3. The attempt at a solution

2. Oct 6, 2007

### Staff: Mentor

No. What makes think KE is conserved in that situation? Do the calculation and find out for yourself.

Well, it's not elastic--they stick together! KE is not conserved.

3. Oct 6, 2007

### ehrenfest

Why does the fact that they stick togethor require that KE is not conserved?

4. Oct 6, 2007

### Staff: Mentor

Just do the calculation! Use conservation of momentum:

$$m_1\vec{v}_1 + m_2\vec{v}_2 = (m_1 + m_2)\vec{v}_f$$

Then compare initial and final KE. Keep it simple if you like, let v_2 = 0 and assume that all motion takes place along a single dimension.

5. Oct 6, 2007

### ehrenfest

OK. I did the calculation and I see that KE gets lost. But, in general, in a closed system, energy must be conserved, so that KE gets transformed to the energy in chemical bonds that holds the two units together, correct?

6. Oct 6, 2007

### Staff: Mentor

When we say KE is "lost" we are talking about macroscopic translational KE. Of course, that energy isn't really lost--it's transformed mainly into thermal energy.