- #1

uchicago2012

- 75

- 0

## Homework Statement

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?

## Homework Equations

f

_{r}= u * N

where f

_{r}= the friction force

u = coefficient of friction

N = the normal force

## 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.