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twotaileddemon

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Well, in any case, I'm having trouble with my homework problem again X_X (specificially c, because it's new). I was wondering if someone could check my work because on problems that are assigned before we go over them in class, I can't seem to figure them out completely. (We just finished another chapter, were given this for homework with no background on torque/center of gravity. I looked it up in my textbook, but am a bit confused. Here is my homework problem:

A box of uniform density weighing 100 N moves in a straight line with constant speed along a horizontal surface. The coefficient of sliding friction is .4 and a rope exerts a force F in the direction of motion. (There is a diagram with a box and an arrow going right with an "F" by it.

a) On the diagram to the right (just a box), draw and identify all the forces on the box.

I have the "F" mentioned in the problem to the right as it was originally drawn, "Ff" to the left, "Fn" up, and "mg" down, which the weight = 100 N

b) Calculate the force F exerted by the rope that keeps the box moving with a constant speed.

Well... I said that F = Ff = uFn

And if Fn = mg, then uFn = umg

So.. F = umg = (.4)(100 N) = 40 N

c) A horizontal force F' is applied at a height 5/3 meters above the surface, is just sufficient to cause the box to begin to tip forward about an axis through point P. The box is 1 meter wide and 2 meters high. Calculate the force F'.

(The diagram is a rectangle - the top and bottom are 1m, the sides are 2m, and on the right (that is, where the F' is) F' is draw to the right 5/3 m above the ground.

I'm pretty sure this involves torque somehow.. if not that, then definitely center of gravity. The only thing is, I'm not sure if I use the forces I gathered in my free body diagram or not. If I do, then F' would have to be slightly above 40 N, but that doesn't seem like the right way to go because I'm given all this other information. Could I use the equation for center of gravity, perhaps? But I only have one mass.. Could someone possibly give me any hint on how to start this? I'm really having trouble getting started. Thanks for any help. I appreciate it immensely.