NDA (II) 2021 General Ability Test · Question 114 of 149

Correct — D, an upward force. Buoyancy is the upward force a fluid exerts on any object placed in it, arising because pressure in a fluid increases with depth, so the push on the bottom of the object exceeds the push on its top. Archimedes' principle fixes its size: the buoyant force equals the weight of the fluid the object displaces. It is a force, measured in newtons, acting vertically upward, not a pressure and not a downward pull.

• (a)upward pressure — Buoyancy does act upward, but it is a net force in newtons, not a pressure; pressure (force per unit area) is the cause, while the buoyant force is the upward resultant of all those pressures.
• (b)downward pressure — This is wrong on both counts — the resultant of fluid pressure on a submerged body points upward, not downward, and buoyancy is a force, not a pressure.
• (c)downward force — Buoyancy opposes an object's weight and therefore acts upward; a downward force would push the object deeper, the opposite of what a fluid does.

Buoyancy, also called up-thrust, is the upward force that a fluid exerts on a body immersed in it. It exists because fluid pressure grows with depth, so the fluid pushes harder on the lower surface of the body than on the upper surface, and the difference is a net upward force.

The word up-thrust is another name for buoyancy, a quick reminder that it acts upward. The trap is the pair of pressure options, because pressure is what creates the force, but the quantity the question asks for is the resultant force, so an upward force is the precise answer.

• Buoyancy, or up-thrust, is the upward force a fluid exerts on a body immersed in it.
• By Archimedes' principle it equals the weight of the fluid displaced by the body.
• It arises because fluid pressure increases with depth, pushing harder on the lower surface.
• A body floats when the buoyant force balances its weight and sinks when its weight is greater.

• Calling buoyancy a pressure when the question asks for a force.
• Assuming the fluid pushes down; the net fluid force on a submerged body is upward.

Asked as a one-line definition; identify buoyancy as an upward force (up-thrust), not a pressure or a downward force.

• practice — not a real PYQ

  Archimedes' principle states that the buoyant force on a body immersed in a fluid equals the

  • (a)weight of the body
  • (b)weight of fluid displaced by the body
  • (c)volume of the body
  • (d)density of the fluid

  Answer(b) weight of fluid displaced by the body — the buoyant force matches the displaced fluid's weight.

• practice — not a real PYQ

  A body floats in a liquid when the buoyant force acting on it is

  • (a)less than its weight
  • (b)equal to its weight
  • (c)zero
  • (d)directed downward

  Answer(b) equal to its weight — balanced forces let the body float.