Figure 10.5b Forces acting on the block in Figure 10.4 , with the weight vector resolved into
components that line up with the friction force and the normal force.
Memorize this
As a rule of thumb, in virtually all inclined-plane problems, you can always break the weight
vector into components parallel and perpendicular to the plane, where the component parallel to
(pointing down) the plane = mg(sin θ) and the component perpendicular to the plane = mg(cos θ).
This rule always works, as long as the angle of the plane is measured from the horizontal.
Even Physics C Students Must Use Free-Body Diagrams
It must be emphasized that even Physics C students must go through the four-step problem-solving process
described in this chapter. Frequently, Physics C students try to take shortcuts, thinking that equilibrium
problems are easy, only to miss something important. If free-body diagrams are good enough for
professional physicists to use, they are good enough for you.
Torque
Torque occurs when a force is applied to an object, and that force can cause the object to rotate.
Torque = Fd
In other words, the torque exerted on an object equals the force exerted on that object (F ) multiplied by
the distance between where the force is applied and the fulcrum (d ) as long as the force acts
perpendicular to the object.
Fulcrum: The point about which an object rotates
Figure 10.6 shows what we mean: