When a coiled spring is pulled, as in Figure 5.1a, the spring stretches. When a
stationary cart is pulled, as in Figure 5.1b, the cart moves. When a football is
kicked, as in Figure 5.1c, it is both deformed and set in motion. These situations
are all examples of a class of forces called contact forces. That is, they involve physical
contact between two objects. Other examples of contact forces are the force
exerted by gas molecules on the walls of a container and the force exerted by your
feet on the floor.
Another class of forces, known as field forces, does not involve physical contact
between two objects. These forces act through empty space. The gravitational
force of attraction between two objects with mass, illustrated in Figure 5.1d, is an
example of this class of force. The gravitational force keeps objects bound to the
Earth and the planets in orbit around the Sun. Another common field force is the
electric force that one electric charge exerts on another (Fig. 5.1e). As an example,
these charges might be those of the electron and proton that form a hydrogen
atom. A third example of a field force is the force a bar magnet exerts on a
piece of iron (Fig. 5.1f).
The distinction between contact forces and field forces is not as sharp as you may
have been led to believe by the previous discussion. When examined at the atomic
level, all the forces we classify as contact forces turn out to be caused by electric
(field) forces of the type illustrated in Figure 5.1e. Nevertheless, in developing models
for macroscopic phenomena, it is convenient to use both classifications of forces.
The only known fundamental forces in nature are all field forces: (1) gravitational
forces between objects, (2) electromagnetic forces between electric charges, (3) strong
forces between subatomic particles, and (4) weak forces that arise in certain radioactive
decay processes. In classical physics, we are concerned only with gravitational and
electromagnetic forces. We will discuss strong and weak forces in Chapter 46.