Momentum question.

Woah dude ever thought of creating one thread and posting all of them in there i've just seen another one of yours I believe only a couple of seconds ago...

as for the question soz im stuck on linear expressions and correlations

For a perfectly elastic collision, speed of approach = speed of separation. The speed of approach is u1+u2 as the spheres are moving towards each other so their velocities relative to each other are the sum of the velocities. After the collision the spheres move in the same direction. We can assume v2>v1 as otherwise the sphere would immediately collide again. Therefore the velocity of the second sphere relative to the first is v2-v1 (as the first sphere is moving in the same direction). So D is the correct answer.

That thread was created 11 hours ago. And I've already asked from a mod about this, so it's okay to make new threads after time intervals.

Let's see if anybody can help us.

just hope someone can help me i have no clue how to do it

There are two ways of solving this question (I mean I know two.)

For elastic collisions, total energy of the system is conserved. Momentum of the system is conserved in any collision. Can you write down the laws of conservation of momentum and energy for these to balls in the form of equations?

Another, quicker approach would be to notice what Hudzy suggests. He says that "speed of approach = speed of separation", or to put it in other words, relative velocity of the two balls is conserved*. This fact follows from the laws of conservation (you should be able to prove it yourself), but is so useful that it is worth remembering that instead of deriving it from a system of equations each time.


*Of course this is only true for perfectly elastic collisions.

Sorry to put you through writing that answer out however I was on about the A level maths questions i am being forced to do for my post grad they are a lot easier than this but, last time I did maths was 4 years ago and never touched on any in my topic including (linear expressions, correlations and standard definitions are just a few) .