# Homework Help: Elastic collision

1. Sep 30, 2010

### bobred

1. The problem statement, all variables and given/known data
This is an exam prep question

Particle A has mass 3m and velocity i + 2j, it collides with particle B mass m velocity -i + j. Collision is elastic. After collision Particle A is observed to be moving along the +ve y-axis. Find the final speeds of the two particles.(two possible cases)

2. Relevant equations
Being elastic the kinetic energy is the same before the collision and after.
We are given the answers ($$3/2,\sqrt{41}/2$$ or $$2,\sqrt{5}$$.

3. The attempt at a solution
Could someone give me a little nudge in the right direction? I can't see where to begin, what is the state of B after the collision?

Thanks

2. Sep 30, 2010

### rock.freak667

Right, so you can form the conservation of kinetic energy equation.

You also need to use conservation of linear momentum. Momentum before = momentum after.

Since you do not know the magnitudes of the final velocities, here is how I put them as

vAj and vBi + βj

3. Sep 30, 2010