# Homework Help: (Conservation of Linear Momentum) Find u1 speed

1. Oct 16, 2011

### paola8

1. A particle of mass m1 = 2 kg moving at speed u1 makes a one-dimensional completely inelastic collision with a particle of mass m2 = 3 kg, intially at rest.

If 60 J of kinetic energy are lost, find u1?

2. Conservation of linear momentum:
m1u1 + m2u2 = m1v1 + m2v2

3. Using the conservation of linear momentun formula...
2u1 + 3(0) = (m1 + m2)V
2u1 = 5V

I'm just not sure where the 60 J of ke lost fits in. The loss in kinetic energy of 60 J means
Kfinal - Kinitial

which is
[.5(m1 + m2)v^2] - [((.5)m1u^2) + ((.5)m2(0)]

But there still remains the unknown variable of u, meaning I went in a circle.

Last edited: Oct 16, 2011
2. Oct 16, 2011

### DukeLuke

You have two equations and two unknowns. There are many ways to solve this. One is way is substituting your answer for v in terms of u1 from the momentum equation into your energy equation. Then solve for u1.

3. Oct 16, 2011

### paola8

Thanks! Once again, i'm left feeling dumb after such a simple solution.