# Grade 12 Physics - mass on an inclined plane (different question)

A block weighing 80N rests on a plane inclined at 20 degrees to the horizontal. The coefficient of static friction is 0.25, while the coefficient of kinetic friction is 0.15.

a) What is the minimum force F, parallel to the plane that will prevent the block from slipping down the plane?
b) What is the minimum force F that will start the blocking moving up the plane?
c) What force F is required to move the block up the plane at constant velocity?

a) I am confused about the difference between quesiton a and b? I don't know how to determine the minimum force that will prevent the block from slipping down the plane.

b) my equations

x-direction: Fapplied - Ffriction - mgsin theta = 0
y-direction: N - mgcos theta = 0
Ffriction = usN
Ff = us (mg cos theta)
Fapplied = us (mg cos theta) + mgsin theta
Fapplied = 18.79 + 27.36 = 46.15 N

N = mgcos theta

c)

Ffriction = ukN
Ff = uk (mgcos theta)
Fapplied = uk(mg cos theta) + mg sin theta
Fapplied = 10.4 + 27.36 = 37.76N

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actually for a:

F = mg (sin theta - ucos theta)

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