# Homework Help: Elastic collision.

1. Nov 23, 2008

### dbb2112

1. The problem statement, all variables and given/known data

A 70kg man who is ice skating north collides with a 30kg boy who is ice skating west. Immediately after the collision, the man and boy are observed to be moving together with a velocity of 2m/s in a direction 37 degrees north of west. What was the magnitude of the boy's velocity before collision.
2. Relevant equations

3. The attempt at a solution
I do not really know where to start. Could someone please list equations that are needed for this problem.

2. Nov 23, 2008

### dbb2112

i meant to title inelastic

3. Nov 23, 2008

### LogicalTime

use conservation of momentum, you should have an equation for the y axis (north/south) and an equation for the x axis (east/west)

4. Nov 23, 2008

### dbb2112

I do not see how to use it when only the final velocity is given.

5. Nov 23, 2008

### dbb2112

ok im not sure if this is right, but i made the equation m2v2cos=m1v1'cos+m2v2'cos
and found v2 to equal 2.13 would that be my final answer?

6. Nov 23, 2008

### LogicalTime

usually good to break these down into momentum in x direction and y direction

$M_{man}V_{man} \hat{y} = M_{final}V_{yfinal} \hat{y}$
$M_{boy}V_{boy} \hat{x}=M_{final}V_{xfinal} \hat{x}$
$M_{final} = M_{boy} + M_{man}$
$V_{yfinal} = \left( 2 \frac{m}{s} \right) cos(37^{\circ})$
see if you can determine the other final V and solve, lemme know if you have questions on notation and such

Last edited: Nov 23, 2008
7. Nov 23, 2008

### dbb2112

Thanks im good now

8. Nov 23, 2008

### LogicalTime

xcellent,

"may entropy increase in a way that favors your sword"
-Logicaltime