Incline block

1. Dec 24, 2007

cryptoguy

1. The problem statement, all variables and given/known data
A block weighing 75 N rests on a plane inclined at 25 degrees. A force F is applied to the object at 40 degrees to the horizontal, pushing it upward on the plane. Coefficient of static friction between block and plane is .363

EDIT: What is the minimum value of F that will prevent the block from slipping down the plane?

2. Relevant equations
Ff = uFn. F = ma, etc.

3. The attempt at a solution

So the free-body diagram has F and Ff in the same direction (diff angles) and F(parallel to surface) is in the other way.

Ff + Fcos(15) = Fgsin(25) (15 degrees because 45-20 = 15)
.363*75cos(25) + Fcos(15) = 75sin(25)

I solved for F and got 7.3 N while the answer is 8.05 N. Thank you for any hints/help

Last edited: Dec 25, 2007
2. Dec 25, 2007

Feldoh

If you have a block moving up a plane then it's kinetic friction (which will most likely be <= static friction) for starters... Also the friction force opposes the movement of the object you it should be

Fcos(15) = Fgsin(25) + Ff

3. Dec 25, 2007

cryptoguy

Wow I forgot to put the actual question down, they're actually asking

What is the minimum value of F that will prevent the block from slipping down the plane?

4. Dec 25, 2007

Staff: Mentor

Hint: What's the normal force? How does F affect the normal force?

5. Dec 25, 2007

cryptoguy

got it thank you.