Page 122 - Physics

Physics for Secondary Schools

Thus,
(
So, V = V + V = V + 8)cm 3
3 1 2 1 ρ × V ρ
(
V = liquid 3 = liquid × V + 8)
The upthrust on the hydrometre when 1 ρ ρ 1
water water
in water is U = ρ × V × g
water 1 0.8
(
V = × V + 8)
Upthrust on the hydrometre when in 1 1.0 1
the liquid U = ρ liquid × V × g V = 0.8V +6.4
3

From the law of fl oatation, 1 1
The weight of the hydrometre = 0.2V = 6.4
1
upthrust in water = upthrust in liquid
V = 32 cm 3
That is 1
ρ × V × g = ρ × V × g Therefore, the v olume of hydrometre
3
water 1 liquid 3 below 1 cm mark is 32 cm .

ICT corner

Visit https://phet.colorado.edu select the simulation titled "Density
and buoyancy" to help students visualise and demonstrate the concept
of denisty of a substance.

Chapter summary

1. An object immersed partially or totally in a fl uid (liquid or gas) experiences an
upward force called the buoyant force or upthrust.

2. The buoyant force causes the object to have an apparent weight that is less than
its weight in the air. Apparent loss in weight = weight in air – weight in a fl uid.
3. Archimedes’ principle states that ‘The upthrust on a body is equal to the weight
of fl uid displaced by the object. That is, upthrust = weight of displaced fl uid.
4. The density and relative density of solid and liquid can be determined using
Archimedes’ principle. The same principle applies whether determining density
or relative density.
Therefore,

For a solid
weight of a solid in air
R.D = ⋅
weight of a solid in air − weight of a solid in water

116
Student’s Book Form One

Physics Form 1 Final.indd 116 16/10/2024 20:57