What Are Forces?
Whenever we lift something, push something, or
otherwise manipulate an object, we are exerting a force. A force
is defined very practically as a push or a pull—essentially it’s
what makes things move. A force is a vector quantity, as it has
both a magnitude and a direction.
In this chapter, we will use the example of pushing a
box along the floor to illustrate many concepts about forces, with
the assumption that it’s a pretty intuitive model that you will
have little trouble imagining.
Physicists use simple pictures called free-body
diagrams to illustrate the forces acting on an object. In
these diagrams, the forces acting on a body are
drawn as vectors originating from the center of the object. Following
is a free-body diagram of you pushing a box into your new college
dorm with force F.
Because force is a vector quantity, it follows the rules
of vector addition. If your evil roommate comes and pushes the box
in the opposite direction with exactly the same magnitude of force
(force –F), the net force
on the box is zero