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Question
A body is held immersed in a liquid. (i) Name the two forces acting on the body and draw a diagram to show these forces. (ii) State how the magnitudes of two forces mentioned in part (i) determine whether the body will float or sink in liquid when it is released. (iii) What is the net force on the body if it (a) sinks and (b) floats?
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Solution
(i) Two forces acting on the body are as listed below:
(a) Weight of the body (downwards)
(b) Upthrust of the liquid (upwards)
(ii) If the weight of the body is greater than the upthrust acting on it, the body will sink
If the weight of the body is equal to or less than the upthrust acting on it, the body will float.
(iii) (a) The net force acting on the body when it sinks is body's own weight.
(b) The net force acting on the body when it floats is the upthrust due to the liquid.
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Assertion: To float, body must displace liquid whose weight is equal to the actual weight.
Reason: The body will experience no net downward force in that case.
