Page 75 - Physics

Introduction to force

Original shape
Compressed

(a) Stretching force on (b) Compressional force on a spring
the rubber band
Figure 3.11: Stretching and compressional forces

Elastic (Restoring) force

Some materials tend to restore their original shapes and sizes when a force acting on
them is removed. Such materials are said to be elastic materials. Examples of elastic
materials are rubber bands and springs. As one stretches or compresses an elastic material,
such as a spring, it resists the change in shape. The material exerts a counter force in the
opposite direction to the stretching or compressing force. This force is called elastic force.
As soon as the stretching or compressing force is removed, the elastic force causes the
material to return to its original shape. Hence, the elastic force is also called a restoring
force. Figure 3.12 illustrates the deformation and restoration of a spring.

Original shape Compressed Restored shape
Figure 3.12: Restoring force of a spring

Air resistance large surface areas. For instance, a runner
Air resistance is the force that resists the has to exert force to oppose the force of
movement of an object through the air. Air a blowing wind. On the contrary, if the
resistance is an example of fl uid resistance runner moves along the direction of the
or fl uid force. It is the force exerted on an wind, the wind provides the force that
object moving through a fl uid (liquid or increases his or her speed. Air resistance
gas). This force depends on the: slows down many moving objects such
(a) size and shape of the object; as a car and even cyclists, as shown in
(b) speed of the object; and Figure 3.13 (a). However, air resistance
(c) density of the fl uid. is advantageous to parachutists since
it acts against the force of gravity on
Air resistance is most noticeable in objects the parachute and slows its downward
moving at high speeds or objects with motion, as shown in Figure 3.13 (b). A

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