It's not the wrong direction. It's just that you've taken the velocity of B after the first collision to be positive towards the left,

For a third collision, A must travel faster than B towards the right. Hence the minus sign on B's velocity.

Reply 4

Zenarthra

Original post by ghostwalker

Ok i understand, but A's velocity doesnt really need to be greater than B's velocity, they could collide like this o> x <O where x is point of impact.
But i understand as the questions asks to show e> something therefore a velocity must be greater than another velocity.

Thanks, need help here please: http://www.thestudentroom.co.uk/showthread.php?t=2690888

Reply 5

ghostwalker

Original post by Zenarthra

Ok i understand, but A's velocity doesnt really need to be greater than B's velocity

Yes, it does need to be.

they could collide like this o> x <O where x is point of impact.

Taking to the right as positive, then A's velocity is +ve, B's is -ve, and A's is therefore greater than B's.

Be careful of confusing velocity and speed.

Reply 6

Zenarthra

Original post by ghostwalker

Yes, it does need to be.

Taking to the right as positive, then A's velocity is +ve, B's is -ve, and A's is therefore greater than B's.

Be careful of confusing velocity and speed.