# Kinetic energy lost during a collision?

1. Dec 8, 2015

### nips

1. The problem statement, all variables and given/known data
A 1kg object traveling at 1 m/s collides head-on with a 2 kg object initially at rest. If the collision is completely inelastic, determine a) the velocity of the objects after impact and b) how much kinetic energy was lost during the collision.

2. Relevant equations
Part a) m1v1 + m2v2 = (m1+m2)v'
Part b) ke before =1/2m1v1^2, ke after= 1/2 (m1 + m2)v2^2

3. The attempt at a solution
Part a)
m1v1 + m2v2 = (m1+m2)v'
1 + 0 = 3(v')
v' = 1/3

Part b)
KE before: 1/2 m1v1^2
.5 (1)(1^2)
=.5
KE after: 1/2 (m1+m2)v2^2
.5(3)(0) = 0??????

KE Lost:
(kinetic before - kinetic after)/kinetic before
(.5-0)/.5 = 1???????

Last edited: Dec 8, 2015
2. Dec 8, 2015

### Mister T

You have the system gaining energy! That can't be right. Check your figures.

Plus, you found the fractional loss, and you were asked for just the loss.

3. Dec 9, 2015

### ehild

Explain what are v1, v2 and v'.

4. Dec 9, 2015

### Epic Mango

The velocities you're using for kinetic energy are wrong; try to use the same variable names as with your momentum equation.