A body sinks in water when
- (a)density of water is greater than density of body
- (b)density of water is less than density of body
- (c)density of water is equal to density of body
- (d)the body is heavy
Correct — B, a body sinks in water when the density of water is less than the density of the body — that is, when the body is denser than water. A submerged body experiences an upward buoyant force equal to the weight of the water it displaces. If the body is denser than water, its own weight exceeds this buoyant force, so the net force is downward and it sinks.
- (a)density of water is greater than density of body — This is the floating condition, not the sinking one. When water is denser than the body, the buoyant force exceeds the body's weight, so the body floats.
- (c)density of water is equal to density of body — When the two densities are equal the body neither sinks nor floats — it stays suspended at whatever depth it is placed (neutral buoyancy).
- (d)the body is heavy — Heaviness alone does not decide it — a huge, heavy steel ship floats while a tiny iron nail sinks. What matters is density (mass per unit volume), not total weight.
Whether a body sinks or floats is decided by Archimedes' principle: compare the body's density with the density of the fluid. If the body is denser than the fluid it sinks; if it is less dense it floats; if the two are equal it stays suspended (neutral buoyancy).
The classic trap is option (d) — the belief that heavy things sink. A ship weighing thousands of tonnes floats because its average density, counting the enclosed air spaces, is less than that of water. Always reason in terms of density, not total weight.
- The upward buoyant force equals the weight of the fluid displaced (Archimedes' principle).
- A body sinks if its density is greater than the fluid's, floats if it is less, and stays suspended if they are equal.
- Iron (about 7.8 g/cm3) sinks in water (1 g/cm3) but floats on mercury (about 13.6 g/cm3).
- A steel ship floats because its average density, including the hollow air spaces, is below that of water.
It is density, not total weight, that decides — a heavy steel ship floats while a light iron pin sinks.
- Thinking heavy objects always sink — it is density, not weight, that decides.
- Forgetting the neutral case, where equal densities leave the body suspended.
Asked as the condition for sinking or floating, or applied to ships, submarines and the classic 'iron floats on mercury but sinks in water'.
Assertion (A): An iron ball floats on mercury but gets immersed in water. Reason (R): The specific gravity of iron is more than that of mercury.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is not a correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer(c) A is true but R is false
Same density-versus-fluid idea. There an iron ball floats on denser mercury but sinks in less dense water (the reason is false because iron's specific gravity is actually less than mercury's); here a body sinks precisely when its density exceeds that of water.
A pumpkin weighs 7.5 N. On submerging it completely in water, ¾ L of water gets displaced. The acceleration due to gravity at the place where the pumpkin was weighed is 10 m/s². Which one of the following is the correct value of the density of the pumpkin?
- (a) 10 kg/m³
- (b) 100 kg/m³
- (c) 1000 kg/m³
- (d) 10000 kg/m³
Answer(c) 1000 kg/m³
Same buoyancy family — Archimedes' principle links the displaced water to the body's density. There you compute the pumpkin's density (1000 kg/m3, equal to water, so it just floats); here the sink/float outcome turns on whether that density exceeds water's.
- practice — not a real PYQ
An iron nail sinks in water but a large steel ship floats because
- (a)the ship is lighter than the nail
- (b)the ship's average density, including its air spaces, is less than that of water
- (c)steel is different from iron
- (d)the sea is denser than a bucket of water
Answer(b) the ship's average density, counting the enclosed air, is less than that of water, so it floats.
- practice — not a real PYQ
A body floating on a liquid displaces a weight of liquid that is
- (a)greater than its own weight
- (b)equal to its own weight
- (c)less than its own weight
- (d)zero
Answer(b) equal to its own weight — this is the law of floatation.