Jamb 1978
A solid suspended by a piece of string is completely immersed in water. On attempting to lift the solid out of the water, the string breaks when the solid is partly out of the water. This is because
A. the tension in the string decreases as the solid is lifted
B. the mass of the solid has increased
C. the solid apparently weighs less when completely immersed in water than when partially immersed
D. part of the solid still in water is exerting more force on the string
E. the solid is no longer in equilibrium with water.
ANSWER: Option C
EXPLANATION: Do you remember Archimedes’ principle?
Archimedes’ principle, physical law of buoyancy, discovered by the ancient Greek mathematician and inventor Archimedes, stating that any body completely or partially submerged in a fluid (gas or liquid) at rest is acted upon by an upward, or buoyant, force, the magnitude of which is equal to the weight of the fluid displaced by the body. The upthrust acts in the opposite direction to the weight, consequently reducing the net effective weight.
When the object is submerged, it weighs less because of the buoyant force pushing upward. The object’s specific gravity is then the object’s weight in air divided by how much weight the object loses when placed in water.
Liquids generally have a higher density than air, so the amount of air displaced by the solid in air has less weight than the liquid of similar volume displaced in the liquid. Therefore upthrust is always less in air than in a liquid.
So, the solid will weigh more in air than when it is partially immersed in water and lastly weigh the least when totally immersed in water.
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