# Question about conservation of momentum

Hi everyone,
If you have a sphere of 10kg, travelling at an initial velocity v. And this sphere collides with two other spheres, each of 5kg. One shoots off at an angle of 60 degrees from the normal, and one at 45 degrees from the normal. There sphere that fires off at 60 degrees now travels at 4 m/s. How can I calculate the initial velocity of the 10 kg sphere?

All momentum is conserved.

Cheers

Is the 10 kg sphere at rest finally? I don't think it can't be done without that fact.
did you try to do the problem yourself?

rcgldr
Homework Helper
Assuming the collision is elastic, then both momemtum and energy are conserved. Before the collision, all of the momentum (mv) and energy (1/2 m v^2) was in the 10 kg sphere.

You also have to assume that the 10kg sphere collided with both the 5kg spheres at the same time. I'm not sure if it works out to be the same if you assume that the collsion with the 45 degree sphere occurred first and then the collsion with the 60 degree sphere second.

Also the problem statement quotes degress from the normal, which if meant literally, means that the 60 degree ball ends up going 30 degrees from the direction that the 10kg ball was going, if "normal" means perpendicular as it usually does.

Last edited:
Harry, draw a picture of the problem. It makes life easter 