Page 118 - Physics

Physics for Secondary Schools

let us infl ate the balloon with helium gas,
which has a density of 0.000178 g/cm .
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Suppose we put 3 000 cm of helium in the
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balloon. The mass of this helium is 0.534 g,
therefore the balloon has a total mass of
6.034 g and a total volume of 3 003.62 cm .
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The density of the helium balloon becomes
0.00200 g/cm , which is still greater than the
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density of air. Let us add 2 500 cm more
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helium to the balloon. Now, the total mass
is 6.479 g and the total volume is 5 503.62
cm . The density of the helium balloon is
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now 0.00118 g/cm , which is less than the
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density of air. The balloon will then fl oat
in the air.

Hot-air balloon Figure 5.11: Hot-air balloon
A hot-air balloon shown in Figure 5.11,
consists of a bag called an envelope that A hot-air balloon can be made to rise or
is capable of containing hot air. Suspended descend by changing the temperature of
beneath the envelope is the gondola, which the air in the envelope. If the temperature
carries a source of heat and passengers. As is increased, the air expands more
the temperature of the air in the envelope resulting to a greater upthrust causing the
increases, it expands. As the air expands, balloon to rise higher in air. If the air in
its volume increases while the mass remains the envelope is cooled, it contracts to a
constant resulting in a decrease in the density smaller volume producing less upthrust
of air in the envelope. This makes the air and thus lowering its height. You should
in the envelope less dense than the cooler note that the rise and fall of the hot-air
surrounding air. This difference in densities balloon is infl uenced by factors such as
produces an upthrust that lifts the balloon the temperature of outside air and the
and its passengers into the air. variation of air density with altitude.

Example 5.8

A balloon made up of a fabric weighing 80 N has a volume of 1.0 × 10 cm . The
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balloon is fi lled with hydrogen of density 0.9 kgm . Calculate the greatest weight in
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addition to that of the hydrogen and the fabric, which the balloon can carry in air of
an average density 1.25 kgm .
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112
Student’s Book Form One

Physics Form 1 Final.indd 112 16/10/2024 20:56