Page 114 - Physics

Physics for Secondary Schools

Exercise 5.2 (f) Calculate the density of the

Whenever necessary use acceleration object.
due to gravity g = 10 N/kg. 3. An object weighs 60 N when in air

1. What volume of iron of density and 40 N when immersed in water.
7.8 g/cm must be attached to wood What is its:
3
whose mass is 100 g and density of (a) Relative density?
0.5 g/cm if both iron and wood have (b) Density?
3
to fl oat under the water surface? 4. A piece of sealing wax of helium
2. An object is hung from a spring weighs 0.27 N in air and 0.12 N
balance. If it weighs 40 N in air and when immersed in water. Deduce:
30 N when immersed in water: (a) its apparent weight in a liquid
(a) Calculate the upthrust on the of density 800 kg/m ; and
3
object. (b) its relative density.
(b) Determine the weight of the 5. A metal cube of side 2 cm weigh
displaced water. 0.56 N in air. Determine:
(c) What is the mass of the displaced
water? (a) its apparent weights when
(d) What is the volume of the immersed in the methylated
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displaced water? spirit of density 0.585 g/cm ; and
(e) Calculate the relative density of (b) the density of the metal of which
the object. it is made.

Law of fl oatation
The previous sections have established that a body can fl oat on a fl uid if certain

conditions are satisfi ed. This leads us to the law of fl oatation which states that:

‘A fl oating body displaces its own weight of the fl uid in which it fl oats’. Activity 5.7
demonstrates the relationship between upthrust and fl oatation.

Acti vity 5.7

Aim: To investigate the relationship between upthrust and fl oatation.
Material: Piece of wood, transparent beaker, water, spring balance, digital
balance, weights
Procedure
1. Suspend a piece of wood from a hook of a spring balance shown in Figure 5.8 (a).
Record its weight,
2. Weigh a dry and empty beaker shown in Figure 5.8 (b).

108
Student’s Book Form One

Physics Form 1 Final.indd 108 16/10/2024 20:56