What is the minimum force needed to make a wheel climb a step?

[Carpe Mori]

Homework Statement

a wheel of mass M has a radius R It is standing vertically on the floor and we want to exert a horizontal force F at its axle so that it will climb a step against which it rests. The step has a height h where h<R what is the minimum force F needed?

Homework Equations

torque = Ia
torque = FR

The Attempt at a Solution

Alrighty so i know that to solve this problem i will have to set the torque created by F equal to torque created by the weight of the wheel (Tf = Tw) and then solve for F but i honestly have no idea how to do this. There was another topic created for this problem but i could not follow what they did =\

https://www.physicsforums.com/showthread.php?t=30938


help please?

 

[Doc Al]

There are only two forces to worry about: Find torques about the pivot point (where the wheel touches the step).

Start by drawing a diagram showing the forces and which way they act.

 

[Carpe Mori]

http://i35.photobucket.com/albums/d185/giddyrayne/Picture018.jpg

I understand torque = perpendicular force * radius

but it seems to me that the force's angle to the pivot point keeps changing.

 

[Doc Al]

but it seems to me that the force's angle to the pivot point keeps changing.

True, but so what? You're asked to find the minimum force that would get it over the step. (At what point is the required force greatest?)

 

[Carpe Mori]

When the torque due to gravity is not greater than the torque due to the force...or i guess the force just has to be equal to it sooo when Tf=Tw?

 

[Doc Al]

At any point, in order to just get the wheel over the step Tf=Tw. The required force will be greatest initially, since that's the point where Tw is greatest. That's all you need to consider to find the minimum force.