Physics elastic collision question help

1. Mar 27, 2009

katrvu

An object with mass 3.6 kg is moving in the positive x direction when it strikea second object head-on. THe collision is elastic and afterwards both objects are moving in the positive x direction. If the final speed of the second object is equal to 8 times the final speed of the first object, what is the mass of the second object?

answer should be 2.62 kg. any one knows how to solve it?

Thank you so much

2. Mar 27, 2009

aniketp

how did you try to approach the problem?

3. Mar 27, 2009

katrvu

well, it says it's elastic collision. so I came up with the following conservations:
let x direction be positive
1. from conservation of momentum
mv1-mv2=mv1f+mv2f
mv1-mv2=mv1f+m8v1f

2. from conservation of energy
1/2mv1^2+1/2mv2^2=1/2mv1f^2+1/2mv2f^2
mv1^2+mv2^2=mv1f^2+m64v1f^2

but there are so many unknows.. donno where to go from here

4. Mar 27, 2009

aniketp

It would be wiser to use the co-effecient of restitution here. The energy equation gets tedious. ;)

5. Mar 27, 2009

Bob S

Is the second object at rest when it is struck by the first object? What quantity has to be conserved in the described collision? What additional quantity is conserved if the collision is elastic (no energy dissipation)?

Next, let's do a thought experiment: Suppose the mass of the second object were the same as the first, what would the results of the collision be? Based on this result, and the description of of the unknown collision, is the mass of the second object more or less than the first?

6. Mar 27, 2009

katrvu

based on the question the 2nd object must be less because it's final speed is much faster, this is because of impulse I believe. The question doesn't say 2nd object is at rest, so 2nd object should have initial speed and collide head on. any suggestions?
thanks

7. Mar 28, 2009

any idea?