# Perfectly Inelastic

Ball 1, with a mass of 100 g and traveling at 10 m/s, collides head-on with Ball 2, which has a mass of 300 g and is initially at rest. What are the final velocities of each ball if the collision is perfectly inelastic?

We're suppose to be talking about Energy, but I figured I could use:

mv+mv (initial)=mv (final)

I got 2.5 m/s for both balls, because if the objects are inelastic they will be traveling at the same speed after collision and momentum is conserved.
Is this right?

## The Attempt at a Solution

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LowlyPion
Homework Helper
That would be correct.

but now I just tried to use conservation of Energy and got a different answer. I tried
1/2 mv^2 + 1/2 mv^2 = 1/2mv^2 (final for system)

I got both balls traveling at 5 m/s ??

LowlyPion
Homework Helper
but now I just tried to use conservation of Energy and got a different answer. I tried
1/2 mv^2 + 1/2 mv^2 = 1/2mv^2 (final for system)

I got both balls traveling at 5 m/s ??
Kinetic energy is not conserved in an inelastic collision.

I tried the V' substitution before but was wrong.

The expression that the book describes it to be:
---> the square root of [(m+M)kd^2] / m^2

You might want to find out the kinetic energy of the balls before the collision, because you are talking about a change in energy.