# Inelastic collision in 2d

1. Jul 16, 2009

### corvus606

1. The problem statement, all variables and given/known data
Two balls od the same masses collide, the first one is moving with the velocity $$v_1$$, the second is stationary. The angle between them is given by the statement: "the direction of the velocity of the first ball forms an angle $$\alpha=30^\circ$$ with the plane of osculation of the balls after the collision"
I have to find magnitude and direction of the velocity after the collision.

2. Relevant equations
$$\vec{p_1}=\vec{p_2}$$

3. The attempt at a solution
$$m\vec{v_1}=(m+m)\vec{v_2}$$
$$\vec{v_2}=0,5\vec{v_1}$$
$$v_{2x}=0,5v_{1x}=0,5v_1cos\alpha$$
$$v_{2y}=0,5v_{1y}=0,5v_1sin\alpha$$

4. The problem
Sounded good to me but the answer is:
velocity perpendicular to the plane: $$0,5v_1sin\alpha$$
velocity parallel to the plane: $$vcos\alpha$$
And I don't have any idea where did I make the mistake :/

2. Jul 17, 2009

### Andrew Mason

Two things are conserved: energy and momentum.

Write out the equations that result from the conservation of energy and momentum. You are given the angle of one ball relative to the direction of the incident ball (30 degrees from that direction. At least that is what I am assuming from the peculiar wording you have given).

AM

3. Jul 17, 2009

### corvus606

The energy is not conserved in the inelastic collision ;)

4. Jul 17, 2009

### Andrew Mason

Then you will have to explain the question better. What exactly happens after the collision?

AM

5. Jul 17, 2009

### corvus606

It isn't clearly stated but most probably the balls are moving "together" (they stick to each other) with a new velocity.