Page 144 - Physics

Physics for Secondary Schools

Pressure due to solids From Figure 7.3 (a) the surface of sides
Pressure in solids depends on the surface 5 cm × 10 cm ×4cm
makes the largest area
of contact. A force (F) applied onto a small of contact.
surface area exerts a greater pressure as Therefore,
compared to when it is applied onto a Largest base area
large surface area. 2
Applied force F = 10 cm ×5 cm = 50 cm
Pressuredue to solid = = ⋅ F
Area of contact A From P =
A
3 N
Example 7.1 P = −3 2
A block of wood weighs 3 N and 5.0 × 10 m
measures 5 cm × 10 cm × 4 cm . If it is = 6.0 × 10 N/m 2
2
placed on a table with its largest base
From Figure 7.3 (b) the surface of sides
area or smallest area face in contact = 4 cm ×5 cm = 20 cm makes the smallest area of
2
with the table, it would exert different contact. Therefore, the smallest base area
pressure on the table. Which surface
would exert the largest pressure on the = 4 cm ×5 cm = 20 cm 2
table? F
But, P =

Solution A
4 cm 3 N
P =
−3
2.0×10 m 2
5 cm
P = 1.5 × 10 N/m 2
3
10 cm The smallest surface area provides
3
(a) Large base area of contact the largest pressure of 1.5 × 10 N/m 2
compared to that exerted by the largest
2
2
surface area of 6.0 × 10 N/m .
Example 7.2

The tip of a needle has a cross-sectional
10 cm
area of 1 × 10 m . If a doctor applies
2
–6
a force of 20 N to a syringe that is
connected to the needle, what is the
pressure exerted at the tip of the needle?

5 cm 4 cm Solution
(b) Small base area of contact Pressure = Normal force
Figure 7.3 Area

138
Student’s Book Form One

Physics Form 1 Final.indd 138 16/10/2024 20:57