This exercise is a bit more complex than the preceding examples. Before beginning the solution, look at all the forces on the sled.
a. First, there is the gravitational force, which always acts downward. This is the weight of the sled. It is labeled mg.
b. The next force to be considered is the normal force. This force always acts perpendicular to the surface, so it pushes against the bottom of the sled. It is labeled FN.
c. The resultant of these forces is a component of the gravitational force that goes in the direction of the motion of the sled, or down the slope. It is labeled fd.
You can redraw these three forces as a right triangle. The angle of the slope corresponds to the angle between mg and FN. Now with the use of trigonometry, you can solve for the force down the incline, Fd.
