A loaded penguin sled weighing 80 N rests on a plane inclined at angle 20 degrees to the horizontal. Between the sled and the plane, the coefficient of static friction is 0.25, and the coefficient of kinetic friction is 0.15.

a) What is the least magnitude of the force F parallel to the plane that will prevent the sled from slipping down the plane?

b) What is the minimum magnitude F that will start the sled moving up the plane?

c) What value of F is required to move the sled up the plane at constant velocity?