Upthrust and flotation
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 1 worked example, 3 practice questions, 4 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Why a steel ship floats
- Where upthrust comes from
- Archimedes' principle
Read the transcript of the teaching part
Last time
Before we start: last time, in Moments and equilibrium, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Why a steel ship floats
A steel nail sinks at once, yet a steel ship weighing thousands of tonnes floats. The steel is the same, the density is the same. What matters is how much water the object pushes aside, and that pushes back up.
Where upthrust comes from
Pressure in a liquid increases with depth, so the bottom of a submerged object feels a bigger pressure than the top. The bottom face is pushed up harder than the top face is pushed down. The difference is the upthrust. For a cube of area A, the bottom is at depth h two and the top at h one. The force difference is rho g A times h two minus h one. A times the difference in depths is the volume of the cube, so the upthrust is rho g V.
Archimedes' principle
Upthrust on an object in a fluid equals the weight of fluid it displaces. Weight of fluid displaced is density of the fluid times the volume of fluid displaced times g. It depends on the density of the fluid, not on the density of the object, and not on how deep the object is. For a fully submerged object, the volume displaced is the volume of the object.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Upthrust · Upthrust.
The whole chapter: Forces, Density and Pressure, Forces, Density and Pressure.
Chapter 4.2: Matter
- Density and pressure in fluids 6 min
- Upthrust and flotation 5 min
Also for A Level Physics 9702
Which topics come up most in A Level Physics 9702 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency.