Page 74 - Physics

Physics for Secondary Schools

It is represented mathematically as:
friction force
Coefficient of friction =
normal reaction (weight )

normal reaction weight of an object= = mg
Figure 3.10
3. Place the setup on a fl at smooth Example 3.2
surface. What is the coeffi cient of friction, which
4. Slowly, pull the spring balance. Keep is calculated in the context of a brick
on increasing the force on the spring with a mass of 8 kg being pulled on a
balance until the brick just start to rough surface with a friction force of
move. Record the reading on the 26 N? (take g = 10 N/kg)
spring balance at the instant the brick
begins to move. Solution
5. Place the setup on a rough surface Mass of a brick = 8 kg
and repeat step 4. Friction force = 26 N

Question Normal reaction ! mg
Compare the magnitude of force = 8 kg×10 N/kg = 80 N
obtained in 4 and 5. Are the values the Coefficient of friction ! friction force
same or different? Give reasons. normal reaction(weight)

26 N
= = 0.325
Coeffi cient of friction 80 N
In activity 3.5, you noticed that more force Therefore, the coeffi cient of friction
is needed to pull the brick placed on the between the brick and the surface is
rough surface than when it is placed on the 0.325.
smooth surface. This is due to the existence
of a large friction on a rough surface than
on a smooth surface. In practice, no surface Stretching and compressional forces
has no friction, rather, some surfaces Stretching force is the force exerted on an
have minimum friction while others have object when its two ends are pulled apart.
maximum friction. On rough surfaces, On the other hand, compressional force
friction is more noticeable because the is the force exerted on the body when its
surface has maximum friction while on two ends are pushed towards each other.
smooth surfaces the friction is minimal. Stretching and compressional forces are
The level of friction caused by a surface important in dealing with elastic materials,
is expressed by the coeffi cient of friction. such as rubber bands, springs, wire and
ropes. Figures 3.11 (a) and (b) show

The coeffi cient of friction is defi ned as the stretching forces on a rubber band and
ratio of friction force to the normal reaction. compressional force on a spring.

68
Student’s Book Form One

Physics Form 1 Final.indd 68 16/10/2024 20:56