Page 102 - Physics

Physics for Secondary Schools

Conditi ons for fl oati ng must be satisfi ed for an object to fl oat in
For an object to fl oat, the upthrust exerted water:
by the surrounding fl uid must be greater (a) The volume of the submerged part
than the objects weight. Upthrust depends of the object must be large enough to
on the volume of the submerged part of the displace a large volume of water.
object and the density of the surrounding (b) The average density of the object must
fl uid. Thus, when an object is placed in a be less than the density of the fl uid in
fl uid, the greater the submerged volume of which it fl oats; and

the object, the greater the upthrust. (c) The upthrust due to water must be
greater or equal to the total weight of
A ship is normally built with a large hollow the object.
space. Typically, the ship has a large volume
which allows it to displace a large volume Task 5.2
of water, resulting into a large upthrust Some water bodies such as the dead
equalising the weight of the ship. If the sea are known to have abnormally high
hollow space is fi lled with air, the average concentrations of salt. Discuss why a
density of the ship material and air is less human body sinks in the normal oceans
than the density of water and so, the ship but fl oats on the dead sea.
fl oats. Therefore, the following conditions

Concept of upthrust

Task 5.3

Hold a stone in one hand and sense its weight. Tie the same stone using a string and
submerge it in water. Hold the string and try to pull it. What do you feel about its
weight?

Consider a cork being held at the bottom of a beaker that contains a liquid, as shown

in Figure 5.3 (a). When the cork is released, it immediately rises to the surface of the
liquid, as shown in Figure 5.3(b). Cork

Hand
Hand

Water
Water Beaker
Cork

(a) (b)
Figure 5.3: Upthrust in water

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Student’s Book Form One

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