# Homework Help: Linear Momentum

1. Oct 12, 2004

### joej

okay well spent all day today working on these problems so my brain is now fried, I got about 2 left out of the 100 I had to do so it's not that bad I guess, but I'm stuck on these two and got no idea where to go.

lets start with the one I got farthest in first (collision question):

1). A battle bot (mass 4.3kg) moving at the speed of 7.8m/s is on a colition course with a second battle bot (mass 5.6kg) moving at the speed of 10.2m/s. They are at a direct right angle to each other, after they collide they stick together. In what direction and with what speed are they (combined mass) moving after the collision?

now okay I do conservation of energy equations, etc. I end up with
5.77=v'sintheta
3.39=v'costheta

I know they are right but..... how the hell do I find out what the angle and velocity is from those 2, I mean I'm pretty sure I have to plug them into one equation, the v' will cancel out and I should get the angle but..... what equation (my math skills are failing me at the moment it seems)

this one I got less far in and I'm sure it's more than a matter of poor math skills

2). A 15g bullet strikes and becomes embedded in a 1.10kg block of wood placed on a table just in front of the gun. if the coefficient of kinetic friction between the block and the surface is 0.25, and the impact drives the block 9.5m before it comes to rest, what was the muzzle speed of the bullet?

so... I calculate the friction force between the blocik withe the bullet embedded and the table Ff=uma which should be Ff=um((v'^2-v^2)/2d))
I'm pretty sure that that's not the way to do this question so.... if someon could point me in the right direction that would be great.

2. Oct 12, 2004

### Staff: Mentor

I assume you mean that you used conservation of momentum. Energy is not conserved.
Assuming these are correct, find theta by dividing one equation by the other. The v' will cancel, and you'll get tan(theta) = some number. (Then use your calculator to find theta.)
Looks like you are trying to calculating the acceleration. That would be a = F/m = umg/m = ug. Now use the kinematic equation to find the speed of the block+bullet after the collision. Then use momentum conservation to figure the initial speed of the bullet.

3. Oct 12, 2004

### joej

thnx all solved ;)