I'm Hopeless

1. Oct 11, 2007

tidalwav1990

I've become completely hopeless. I've started honors physics but fell behind way too quick and have been stringing along for a month now since vectors began being taught. This is abnormal for me as everything has always come easy for me. Our teacher is no help and I'm looking for a tutor. I start my question like this because I need simple answers and any side info on basic physics is greatly appreciated. I also become heavily confused when graphs and x,y coordinates become in use.

1. The problem statement, all variables and given/known data
The coefficient of static friction between the 3.00 kg crate and the 35.0 degree incline is .300. What is the magnitude of the minimum force, F, that must be applied to the crate perpendicular to the incline to prevent the crate from sliding down the incline.

2. Relevant equations

F=M*A---------------Fs=Ms*N
F=force--------------Fs=Force of static friction
M=mass--------------Ms=Coefficient of static friction
A=acceleration--------N=Normal Force

3. The attempt at a solution

i'm so sorry, i don't even know where to begin.
By the way I know the answer is 32.2N

Last edited: Oct 11, 2007
2. Oct 11, 2007

Staff: Mentor

With a mass on an incline, the weight (mg) points vertically downward. This force (vector) can be resolved into one component parallel with the incline and component normal to the incline. When friction is present, it opposes the direction of motion, so the friction component points up the incline.

Now the friction force is proportional to the normal force of an object and that proportionality constant is the coefficient of friction $\mu$.

If the friction force due to the normal component of the weight is less than the parallel component the of the weight, it will slide. Then one has to add another normal force to increase the friction and hold the crate in place.

This reference might help - http://hyperphysics.phy-astr.gsu.edu/hbase/N2st.html#c2

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