Page 123 - Physics

Sinking and fl oating

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

5. An object fl oats or sinks depending on its density and the density of the fl uid in
which it is placed.
6. An object will fl oat if it displaces a weight of fl uid equal to its own weight. In
other words, an object will fl oat if its density is less than the density of the fl uid
in which it is immersed.
7. Hot-air balloons, submarines, ships and weather balloons can be made to fl oat by
altering their volume to make them less dense than the fl uid in which they fl oat.
8. A hydrometre is a device used to measure the relative density of different liquids.
It works on the principle that the hydrometre sinks until it displaces a volume
of liquid that has the same weight as itself. The hydrometre sinks further in low
density liquids than in high-density liquids.

Revision exercise 5 then, the percentage of water by
volume in the diluted milk is:
Section A
(a) 55%.
1. Choose the most correct answer. (b) 40%.
(i) The same body is immersed in liquid
A and then, in liquid B. The extent (c) 50%.
to which the body sinks in liquid (d) 45%.
B is less than in liquid A. What
conclusion can be drawn from such (iii) A student lowers an object into a
an observation? The density of: liquid fi lled in a container. He fi nds

(a) the liquid A is more than that of that there is a maximum apparent
loss in weight of the object when:
liquid B.
(b) the liquid B is more than that of (a) it just touches the surface of the
liquid
liquid A.

(c) the solid body is less than that (b) it is partially immersed and
of the liquid in both cases. also touches the sides of the
(d) the liquid A is the same as that container.
of liquid B. (c) it is completely immersed in the
(ii) A sample of milk diluted with water liquid.
has a density of 1 040 kg/m . If pure (d) it is partially immersed in the
3
milk has a density of 1080 kg/m , liquid.
3

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Physics Form 1 Final.indd 117 16/10/2024 20:57