# Conservation of Momentum law problem

1. Nov 25, 2014

### Quantum Fizzics

1. The problem statement, all variables and given/known data
m1= 3kg m2= 6kg
v1 = 5m/s v2= 2m/s
d = 3m
DIAGRAM : http://gyazo.com/17b3126726b109256a35f65b86708bb0
2. Relevant equations
Conservation of momentum:
m1+v1 = m1v1'+m2v2'

Conservation of elastic:
m1v1^2+m2v2^2 = m1v1'^2+m2v2'^2
3. The attempt at a solution
Now my attempt was use both equations & the answers I've gotten so far are 27=3v1'+6v2'(for conservation of momentum) & the 2nd one I've got for the conservation of elastic is 99= 3v1'^2 + 6v2'^2 since they will collide at some point. The only thing I'm stuck at is what to do next afterwards. My teacher has done a similar question but I can never understand how he did it.

2. Nov 25, 2014

### Maged Saeed

Is the conservattion of momentum law correct ??!
I mean Is it llike what you wrote or like the following
m1v1 + 0 = m1v1+m2v2

Zero indecates that the momentum is zero because the vilocity of the second ball before collision is zero ..
Am I right ..

3. Nov 25, 2014

### Maged Saeed

Sorry v1 has a value
so the will be
m1v1 +m2v2 = m1v1+m2v2

4. Nov 25, 2014

### Quantum Fizzics

Im just stuck because the example that my teacher gave was like "50=2(10-3v2'/2)^2+3v2'^2" that bracket part confuses me

5. Nov 25, 2014

### Maged Saeed

Try to equte the two equation or write v1 in term of v2..

6. Nov 25, 2014

### Quantum Fizzics

WOW WHAT THE ****, OK I DID THAT ONE BEFORE. Its this right? " V1+V1' = V2+V2'" but can u explain why that is that cus it made some sense to me. & I showed my friend this process but, she said it made no sense so.

7. Nov 25, 2014

### Maged Saeed

I think your teacher has written v1 in term of v2
try to do so from the first equation your v1 will be (27 - 6 v2) /3 which is 9-2v2

It seems reasonable
;)

8. Nov 25, 2014

### Quantum Fizzics

but why is that though? :( pls

9. Nov 25, 2014

### Maged Saeed

Because v1 and v2 are related by these two equations and their value must satisfy both of them
so we make this substitution

10. Nov 25, 2014

### Quantum Fizzics

alright thaanks dawg really appreciate that, last question in the example he gave so the last line was 50-50 = -30v2'+7.5v2'^2
the left side what if it wasnt 0

Last edited: Nov 25, 2014
11. Nov 25, 2014

### Maged Saeed

If the lift side isnt zero move it to the right side and solve the quadratic equation..

By the way what does dawg mean in your last comment

12. Nov 26, 2014

### Quantum Fizzics

means like dude lol kids these days