Angular Momentum and Torque Problem

[Joules6626]

Homework Statement

Hi, I'm having a problem with a mechanics question. I'm not really sure where to begin solving the problem. It reads:

A lawn roller is 20 inches in diameter and weighs 100 pounds. What horizontal force is needed to pull it over a brick 2 inches high?
What is the force of the brick on the roller and what direction is it in?

Any help would be so greatfully appreciated. Thanks in advance.

Homework Equations

L = r x p
torque = F x r
d:/dt = torque
Moment of inertia = 1/2MR^2

The Attempt at a Solution

I tried finding the angular momentum about the corner of the brick where the roller first touches it. This would make my R equal to the radius of the roller. But when I find the L when the roller is on the brick, it ends up being the same which can't be true since there is an outside force working on it.

 

[ehild]

Show your work in detail, with a picture attached.ehild

 

[Joules6626]

[PLAIN]http://img842.imageshack.us/img842/8718/42780714.png
I think I might be on to something for the force of the brick on the roller, but I get a d/dt in there and I'm not sure what to do.
The L was taken about the axis on the bottom left of the brick.
As for the force the roller needs to get over the brick, I'm pretty clueless.

 

[ehild]

I don't follow you. Is v the velocity of the ball? You need to find what horizontal force is needed to lift the ball from the ground and make it roll onto the brick. So this is rather a static problem. The ball rotates around the top left corner of the brick. If you calculate the torque around the bottom left corner you have even the torque of the normal force around the top corner take into account.

ehild

 

[Joules6626]

The v is tangential velocity of the roller.
L = r x p = r*p_perpendicular

So I guess the bottom left corner is not the way to go?

 

[ehild]

Why did you choose it? It has no advantage.

ehild

 

[Joules6626]

I wasn't sure where to begin the problem, so I just picked the bottom left corner out of random. I'm still stuck on this problem. If you could help me I'd gladly appreciate it.

 

[ehild]

Write the equation for the torques of both forces around the top left corner assuming the ball just does not touch the ground. Try to find out how the torques change if the ball turns around.

ehild

 

[Joules6626]

[PLAIN]http://img3.imageshack.us/img3/1496/96925170.jpg

So I used the equation T = RFsin(theta) for both torques.
My answer sounds pretty reasonable.
Is the force of the brick on the roller just the normal force? (100N) or is there something more to it?

 

[ehild]

It is the normal force but you need to define its direction and magnitude. Use that the resultant force should be zero. The normal force is the interaction force between the edge of the brick and the ball. You can not define the normal of the edge, but you can define the normal of the ball.

ehild