All objects experience a buoyancy when they are immersed in a fluid. Buoyancy is
- (a)a downward force
- (b)a downward pressure
- (c)an upward force
- (d)an upward pressure
Correct — C, an upward force. When an object is immersed in a fluid, the pressure on its lower surface is greater than on its upper surface, because pressure increases with depth. This pressure difference produces a net upward push called buoyancy, or the buoyant force. It is a force (measured in newtons) directed upward — equal to the weight of the fluid displaced (Archimedes' principle) — not a pressure and not directed downward.
- (a)a downward force — Buoyancy acts upward, opposing gravity; it is what makes objects feel lighter in water, so it cannot be downward.
- (b)a downward pressure — Buoyancy is neither downward nor a pressure; it is an upward force arising from the pressure difference across the object.
- (d)an upward pressure — The direction (upward) is right, but buoyancy is a force in newtons, not a pressure — it is the net result of the pressure difference, so 'upward force' is the precise answer.
The buoyant force (upthrust) is the net upward force a fluid exerts on any body immersed in it. It arises because fluid pressure increases with depth, so the upward push on the bottom of the object exceeds the downward push on the top. By Archimedes' principle its magnitude equals the weight of the fluid displaced.
The tempting distractor is 'upward pressure' — the direction is right, but the quantity is wrong. Buoyancy is a force (newtons), the resultant of a pressure difference, so the exact description is 'an upward force'.
- The buoyant force acts vertically upward on any body immersed in a fluid.
- Its magnitude equals the weight of the fluid displaced (Archimedes' principle).
- It arises because fluid pressure increases with depth, giving more upward push below than downward push above.
- An object floats if the buoyant force is at least equal to its weight; otherwise it sinks.
- Choosing 'upward pressure' — buoyancy is a force, not a pressure.
- Thinking buoyancy acts downward.
A one-line definition item — buoyancy is an upward force, set against the 'upward pressure' distractor.
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 principle — buoyancy and floating. An iron ball floats on denser mercury but sinks in less-dense water; the reason is false because iron's specific gravity is less than mercury's, not more.
Buoyancy is a/an
- (a) upward pressure
- (b) downward pressure
- (c) downward force
- (d) upward force
Answer(d) upward force
The identical concept — buoyancy is an upward force, with the same 'upward pressure' distractor to trap those who confuse force with pressure.
- practice — not a real PYQ
The buoyant force on a submerged body equals the
- (a)weight of the body
- (b)weight of the fluid displaced
- (c)volume of the body
- (d)density of the fluid
Answer(b) weight of the fluid displaced — Archimedes' principle.
- practice — not a real PYQ
An object floats in a liquid when the buoyant force is
- (a)less than its weight
- (b)equal to or greater than its weight
- (c)zero
- (d)always directed downward
Answer(b) equal to or greater than its weight.