Page 125 - Physics

Sinking and fl oating
that help them control their
buoyancy.
(c) Fish secrete oils that reduce their
density and allow them to fl oat.
(d) Fish use their fi ns to anchor
themselves to the bottom of
the water.
Section B
2. Explain the following scenario.
(a) You observe a stone and a leaf
placed in a river. One sinks
rapidly, and the other gently
fl oats on the water’s surface.
Explain the factors responsible
for this difference in behaviour.
(b) You observe that an object
submerged in water appears to
weigh less than its actual weight.
Describe the force responsible
for this difference in weight.
(c) You have a book with a mass of
3 kg. Describe how the force of
gravity determines the book’s
actual weight on Earth.
3. When an object with a mass
of 250 g is submerged in water, its
weight is measured to be 2.2 N.
What is the upthrust and the density
of the object?
4. A ship has a mass of 1 000 tones
and floats in seawater of density
3
1 023.6 kg/m . Calculate the amount
of water displaced.
5. When an object of mass 250 g and
density 8 000 kg/m is submerged
3
in methanol, which has a density
of 0.79 g/cm , what would be its
3
Physics Form 1 Final.indd 119
apparent weight? Use g = 10 m/s 2
6. A 300 cm object weighs 2.5 N in
3
air and 2 N in an unknown liquid.
What is the density of the unknown
liquid?
7. (a) A student places a hollow
plastic ball and a solid plastic
ball of the same size into water.
The hollow ball fl oats, and the
solid ball sinks. Explain the
difference in their behaviour
using Archimedes’ principle
and the concept of density.
(b) Water ice has a density of
0.91 g/cm³, so it will fl oat in
liquid water. Imagine you have
a cube of ice, 10 cm on a side.
(i) What is the cube’s weight?
(ii) What volume of liquid water
must be displaced to support
the fl oating cube?
(iii) How much of the cube is under
the surface of the water?
8. Objects 1 and 2 are placed on
opposite ends of a beam balance as
shown in Figure 5.18. When held
in air the two objects are seen to
have the same weight. When lowered
in water the objects are no longer
balanced.
1 2 1
Air 2
Water
Figure 5.18
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