# Homework Help: Help test tmrw! elastic collision and conservation of momentum problem?

1. Dec 18, 2012

### nchin

Block 1 moves with speed of 10m/s to right. It hits block 2 which has twice the mass of block 1 and speed of 5m/s to right. compute the magnitude and direction of block 1 for a perfectly elastic collision.

solution:

u1 = 10m/s
u2 = 5m/s

m1v1 + m2v2 = m1u1 + m2u2 ---->
v1 + 2v2 = u1 + 2u2
............ = 10 + 2(5) = 20

(1) v1 + 2v2 = 20

why did u2 and v2 get multiplied by two and why did the m cross out?

2. Dec 19, 2012

### doub

I not certain but I think I would go about solving it like this;

The formula you want to use is v'A = ((mA-mB)/(mA + mB))vA

And look at the frame set so that the total velocity of block 1 is actual 10 m/s - 5 m/s so the sped of block one would be 5 m/s.

No guarentee though

3. Dec 19, 2012

### ehild

The second block has twice as much as the first one. So you can write for the mass of the first block "m" and "2m" for the mass of the second one. As all terms in the equation have the common factor m you can cross it out.

ehild

4. Dec 19, 2012

thanks!