Further Mechanics Question

When two spheres collide in two dimensions, why is it necessary that they have equal radii to solve it at A-level (what are we ignoring)? Thanks!

If they don't have equal radii then at collision the line between the centres can only be properly represented in 3 dimensions, which is a complication they don't want you to have to deal with.

Thanks for this. So if I have to explain how the I've used the fact that the spheres have equal radii in my answer, I would just say that equal radii means the line of centres is parallel to the surface on which the particles collide (does this affect the normal reaction force too?)

If they don't have equal radii then at collision the line between the centres can only be properly represented in 3 dimensions, which is a complication they don't want you to have to deal with.

Sorry do you mind explaining this? xx

The centres of the spheres will be at different heights, so the line joining the centres will be at an angle to the plane, hence the need for three dimensions rather than just two. It also follows that the impulse between the spheres, which acts along the line of centres, will be at an angle to the plane, and since the plane doesn't move, there will be an impulse between the spheres and the plane - starting to get a little bit complicated.