(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

A 6 kg object sits on a horizontal surface with coefficient of friction = .8. What is the minimum force (not necessarily horizontal) necessary to make the object slide?

2. Relevant equations

f_{r}= u * N

where f_{r}= the friction force

u = coefficient of friction

N = the normal force

3. The attempt at a solution

I knew there was something more to this problem- In lecture, my prof talked about a "magic" angle theta in the positive x and positive y direction that would take the least amount of force to move an object is one was pulling on the object at said "magic" angle. Since no value was mentioned, I assume the angle varies with the mass and the force being applied to the object.

As usual for me, I have an apparently sound free body diagram, apparently sound equations, and too many unknowns. I have normal force N, force of gravity mg, friction force f_{r}, and the force on the object F.

F_{net,x}= F cos theta - f_{r}= ma

(my prof doesn't believe in the difference between F_{k}and F_{s}(static and kinetic friction) so when a = 0 is when the minimum force is being exerted to make the block move)

F_{net,x}= F cos theta = f_{r}

F_{net,y}= N - mg + F sin theta = ma

acceleration is zero in the y direction so:

F sin theta = mg - N

I can sub in u * N for f_{r}and set tan theta equal to (mg - N) / (u * N), but I don't know N or theta and I'm trying to get to F.

Am I approaching these problems wrong or do I just keep getting lost along the way? I feel like I keep getting stuck in the same places.

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# Homework Help: Forces and friction

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