Page 150 - Physics

Physics for Secondary Schools

Task 7.2
Surface
Fill a polythene bag with water
and hold or press the upper part.
Using a pin, prick the polythene Top
bag randomly. Discuss your Increasing Side Side

observation. pressure Bottom Upthrust

Pressure in liquid and upthrust Object

When an object is submerged
in a liquid, its bottom side
experiences a greater pressure Figure 7.16: Variation in pressure producing upthrust
than the top side. The difference
in pressure between these sides The pressure on the right side of the object
results in an upward force known increases with increasing depth but is exactly

as upthrust. Figure 7.16 shows an balanced by pressure on the left side so that the
object submerged in liquid. net force in the horizontal direction is zero.

Example 7.3

A cube of sides 2 cm is completely submerged in liquid so that the bottom of
the cube is at a depth of 10 cm. If the density of the liquid (ρ) is 1 027 kg/m ,
3
and g = 10 N/kg:
(a) What is the difference between the pressure at the bottom of the cube and the
pressure at its top?
(b) Determine the difference in the forces at the top and bottom.

(c) What is the weight of the water displaced by the cube?

Solution
(a) The bottom of the cube is at a depth of 10 cm = 0.1 m.

Pressure at the bottom = hρg
=0.1 m × 1 027 kg/m × 10 m/s 2
3

= 1027 N/m 2
The top of the cube is at a depth = 0.1m−0.02m = 0.08 m.

2
3
Pressure at the top = 0.08 m ×1 027kg/m ×10 m/s = 821.6 N/m 2

144
Student’s Book Form One

Physics Form 1 Final.indd 144 16/10/2024 20:57