S E C T I O N 5 . 1 • The Concept of Force
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Another class of forces, known as field forces, do not involve physical contact be-
tween two objects but instead act through empty space. The gravitational force of at-
traction between two objects, illustrated in Figure 5.1d, is an example of this class of
force. This gravitational force keeps objects bound to the Earth and the planets in or-
bit around the Sun. Another common example of a field force is the electric force that
one electric charge exerts on another (Fig. 5.1e). 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) nuclear forces between sub-
atomic particles, and (4) weak forces that arise in certain radioactive decay processes. In
classical physics, we are concerned only with gravitational and electromagnetic forces.
Measuring the Strength of a Force
It is convenient to use the deformation of a spring to measure force. Suppose we apply
a vertical force to a spring scale that has a fixed upper end, as shown in Figure 5.2a.
The spring elongates when the force is applied, and a pointer on the scale reads the
value of the applied force. We can calibrate the spring by defining a reference force
F
1
as the force that produces a pointer reading of 1.00 cm. (Because force is a vector
Field forces
Contact forces
(d)
(a)
(b)
(c)
(e)
(f)
m
M
– q
+ Q
Iron
N
S
Figure 5.1 Some examples of applied forces. In each case a force is exerted on the ob-
ject within the boxed area. Some agent in the environment external to the boxed area
exerts a force on the object.