You can push a 335 N trunk up a 20.0° inclined plane at a constant velocity by exerting a 209 N force parallel to the plane's surface. What force must be exerted on the trunk so that it would instead slide down the plane with a constant velocity?
In the initial case, when the body is sliding up, the friction (f) should act in the downward direction along the incline.
by applying Freebody diagram along the incline, we have
F - [ f + Mg sin (Theta) ] = M(Zero) ...
This solution helps go through how to calculate the force exerted for constant velocity of a truck.