Page 119 - Understanding NCERT Science 09

exert pressure on the base and walls of the water on the bottle is greater than its weight.
container in which they are enclosed. Pressure Therefore it rises up when released.
exerted in any confined mass of fluid is To keep the bottle completely immersed,
transmitted undiminished in all directions. the upward force on the bottle due to water
must be balanced. This can be achieved by
9.5.2 BUOYANCY an externally applied force acting downwards.
This force must at least be equal to the
Have you ever had a swim in a pool and felt difference between the upward force and the
lighter? Have you ever drawn water from a weight of the bottle.
well and felt that the bucket of water is heavier The upward force exerted by the water on
when it is out of the water? Have you ever the bottle is known as upthrust or buoyant
wondered why a ship made of iron and steel force. In fact, all objects experience a force of
does not sink in sea water, but while the same buoyancy when they are immersed in a fluid.
amount of iron and steel in the form of a sheet The magnitude of this buoyant force depends
would sink? These questions can be answered on the density of the fluid.
by taking buoyancy in consideration. Let us
understand the meaning of buoyancy by 9.5.3 WHY OBJECTS FLOAT OR SINK
doing an activity.
WHEN PLACED ON THE SURFACE OF
Activity ______________ 9.4 WATER?

• Take an empty plastic bottle. Close Let us do the following activities to arrive at
the mouth of the bottle with an an answer for the above question.
airtight stopper. Put it in a bucket
filled with water. You see that the Activity ______________ 9.5
bottle floats.
• Push the bottle into the water. You feel • Take a beaker filled with water.
an upward push. Try to push it further • Take an iron nail and place it on the
down. You will find it difficult to push surface of the water.
deeper and deeper. This indicates that • Observe what happens.
water exerts a force on the bottle in the
upward direction. The upward force The nail sinks. The force due to the
exerted by the water goes on increasing gravitational attraction of the earth on the
as the bottle is pushed deeper till it is iron nail pulls it downwards. There is an
completely immersed. upthrust of water on the nail, which pushes
• Now, release the bottle. It bounces it upwards. But the downward force acting
back to the surface.
on the nail is greater than the upthrust of
• Does the force due to the gravitational water on the nail. So it sinks (Fig. 9.5).
attraction of the earth act on this
bottle? If so, why doesn’t the bottle stay
immersed in water after it is released?
How can you immerse the bottle in
water?

The force due to the gravitational
attraction of the earth acts on the bottle in
the downward direction. So the bottle is
pulled downwards. But the water exerts an
upward force on the bottle. Thus, the bottle is
pushed upwards. We have learnt that weight
of an object is the force due to gravitational
attraction of the earth. When the bottle is
Fig. 9.5: An iron nail sinks and a cork floats when
immersed, the upward force exerted by the placed on the surface of water.
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