Force
A force is a push or pull. It tends to produce motion
or a change in motion. Force is what makes something
start to move, speed up, slow down, or keep moving
against resistance (such as friction). A force may act on
an object without being in direct contact with it. The
most common example of this is the pull of gravity.
Forces are usually expressed in terms of weight units,
such as pounds, tons. or ounces.
Figure 12-4 shows the action of a force on a body.
An arrow pointing in the direction of the force is drawn
to represent the force. The location and direction of the
force being applied is known as the line of action. If a
number of forces act together on a body, they may be
considered as a single combined force acting in the
same direction to produce the same overall effect. In
this manner you can understand that F4 in figure 12-4
is the resultant or the sum of the individual forces F1,
F2, and F3.