Answer: Pressure increases with depth, so average pressure × area gives total force.
- A Pressure increases with depth, so average pressure × area gives total force
- B Water at every depth pushes on the wall with exactly equal pressure
- C Temperature variation with depth is what raises the hydrostatic pressure
- D The flow velocity of standing water increases with depth
Correct answer: A. Pressure increases with depth, so average pressure × area gives total force
Explanation: Pressure P = ρgh increases linearly with depth h. Total force = ∫ P dA. For a wall of width w and height H: F = ρg(H/2) × wH = ρgwH²/2.
Since the same volume of fluid must pass every cross-section per second (continuity), the fluid speeds up where the pipe narrows; Bernoulli's equation then says this faster-moving fluid has lower pressure - the principle behind a venturi meter and an aircraft wing's lift.
Concept context
Pressure, buoyancy, Bernoulli equation, viscosity, and surface tension.