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All 14 Flashcards — Buoyancy & Archimedes' principle
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Question
State Archimedes' principle.
Answer
The **buoyancy (upthrust) force** on an object equals the **weight of the fluid it pushes aside** (displaces).
Question
What is buoyancy (upthrust)?
Answer
The **upward force** a fluid exerts on an object, because the fluid presses harder underneath than on top.
Question
Formula for the buoyancy force?
Answer
$F_b = \rho V g$ — fluid density × displaced (submerged) volume × g. **Given** in the data booklet.
Question
In F_b = ρVg, whose density is ρ?
Answer
The **fluid's** density — not the object's.
Question
In F_b = ρVg, what is V?
Answer
The **submerged** volume — the volume of fluid pushed aside.
Question
When does an object float?
Answer
When it is **less dense** than the fluid, so the buoyancy can balance its weight.
Question
Condition for a floating object (equilibrium)?
Answer
Buoyancy = weight: $\rho_{fluid} V_{sub}\, g = \rho_{obj} V_{total}\, g$.
Question
Fraction of a floating object that is submerged?
Answer
The **density ratio**: ρ_object ÷ ρ_fluid.
Question
Formula for density?
Answer
$\rho = \dfrac{m}{V}$ — mass ÷ volume. **Given** in the data booklet.
Question
Same fluid, two objects of different size — how do their upthrusts compare?
Answer
Buoyancy ∝ submerged volume (F_b = ρVg), so the ratio of upthrusts = ratio of submerged volumes.
Question
Why is most of an iceberg underwater?
Answer
Ice (≈9.2 × 10²) is only slightly less dense than seawater (≈1.03 × 10³), so the submerged fraction ≈ 0.89.
Question
Common buoyancy mistake to avoid?
Answer
Using the **object's** density for ρ, or the **whole** volume when only part is submerged.
Question
Why does a fluid push up on an object in it?
Answer
The fluid presses harder lower down, so the push on the bottom is bigger than the push on the top.
Question
A cylinder is lowered into a liquid on a force sensor. What do the intercept and gradient of F against submerged length give?
Answer
Intercept = the weight; gradient = −ρAg, so ρ (liquid) = |gradient| ÷ (A g).
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Full study notes for Buoyancy & Archimedes' principle
Topic 1.2 hub
Forces and momentum
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