<p>OK, I'm not sure where to put this, but right now I am in Physics and I have two questions which I am little confused about. I realize these are really simple questions, but here goes:</p>
<p>Both questions deal with free body diagrams and forces. In the first one, there is a box sitting on a floor and I need to know the minimum horizontal force needed to move the box. The box is 40 kg and the coefficient of static friction is .65.</p>
<p>There is an equation, F(s,max)=µ(s)·F(n), where F(n) is force normal and µ(s) is the coefficient of static friction. I know that the sum of the forces in the y-direction is F(sum of y)=F(n)-F(w)=0, where F(w) is the force of weight; therefore F(w)=F(n); and since F(w)=m·g, then F(n)=m·g. Thus, my final formula is F(s,max)=µ(s)·m·g, correct?</p>
<p>My confusion lies in that I set up my sum of forces in the x to be F(sum of x)=F(s,max)-F(f)=m·g, where F(f) is the force of friction. I put that they are equal to m·g, since this is the force needed to put the box into motion, so the box is accelerating, correct? By definition F(f)=µs*F(n). Then, solving for my **F(s,max), I get **F(s,max)=F(f)+mg, or, substituting in friction and the normal force, **F(s,max)=µ(s)·mg+mg*. This equation is different from the one I gave at first, which is from the definition. We are required to use a sum of forces statement and base everything off that, but it seems as if I am making an incorrect assumption, because I wouldn't get the same answer. I hope this made any sense, because I'd really like to figure this out...</p>
<p>My second question is more conceptual... We are given that a block is sliding at a constant velocity down an inclined play at 37°. I need to find the coefficient of kinetic friction. I don't know universal this is, but in our class, we set the direction in which the object is moving to be the parallel direction and the then the direction perpendicular to that direction is the perpendicular direction (confusing, I know ;)). So F(sum of parallel)=F(parallel)-F(f)=ma. Basically, the sum is the force forward it is experiencing due to gravity minus the the friction force. The thing is, it sayas that the box is moving at a constant velocity, and therefore its acceleration in the parallel direction is 0. Which would mean F(parallel)-F(f)=m·0, or F(parallel)=F(f), which would then mean that the friction force equals the parallel force and that the box is in equilibrium and is therefore not moving. Am I completely confused here? I appreciate any help :)</p>