S3 Science: Pressure, Upthrust And Floating (Paper 4 (error Clinic))

S3 Science: Pressure, Upthrust and Floating (Paper 4 (error clinic))

2026-11-14

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Tutor Prep Guide · Pressure as force over area, pressure in liquids changing with depth, atmospheric pressure and the ten metre water column, the principle of upthrust and displaced fluid, density and floating, apparent weight and displacement

S3 Science: Pressure, Upthrust and Floating (Paper 4 (error clinic))

This paper corrects ten common errors on pressure, upthrust and floating, covering the pressure formula, the origin of upthrust, the conditions for floating and sinking, liquid pressure against container shape, the density test, upthrust in gases, apparent weight, the pressure under a needle tip, and the real reason water rises in a straw.

Direct answers　Pressure is force over area; upthrust is the weight of displaced fluid; upthrust equals weight when floating; upthrust is less than weight when sinking; liquid pressure ignores container shape; density decides floating; gases give upthrust; apparent weight is real weight minus upthrust; a small area gives high pressure; atmospheric pressure lifts water in a straw.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	20
Difficulty mix	Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1
Chapter	Chapter: S3 Science: Pressure, Upthrust and Floating
Level	S3

02　Questions (10 items)

FoundationWhat is the formula linking pressure, force and area, and why is pressure not force times area?
FoundationWhat determines the size of upthrust?
FoundationFor an object floating at rest, how does upthrust compare with its weight?
IntermediateWhy is the upthrust on a sinking iron block always smaller than its weight?
IntermediateDoes pressure in a liquid depend on the shape of the container or the total amount of liquid?
IntermediateWhat is the quickest test for whether an object floats in water?
ChallengeWhy do we say air also produces upthrust? Give one example.
ChallengeA metal block weighs 5 N in air and the balance reads 4.2 N when it is in water. What is the upthrust?
ChallengeWhy does a needle pierce paper while a finger pressing on the same spot does not?
Error redoA student says water rises in a straw because the mouth pulls it up. What is the correct explanation?

03　Answers and one-line notes

#	Answer	One-line note
1	Pressure equals force divided by area; a larger area spreads the same force so the pressure is smaller	Multiplying would make a larger area give a larger pressure, which is the opposite of what happens.
2	The weight of the fluid displaced, that is displaced volume times fluid density times g	Upthrust depends on how much fluid is pushed aside, not on the weight of the object.
3	They are equal, so the object neither rises nor sinks	Resting on the surface means the upward upthrust balances the downward weight.
4	Because the upthrust cannot balance the weight, so the resultant force acts downward and the block sinks	If upthrust were greater than or equal to the weight the block would rise or stay suspended.
5	No, it depends only on depth, density of the liquid and g	At the same depth a wide tube and a narrow tube have the same pressure.
6	Compare its density with that of water: denser sinks, less dense floats	Density is mass per unit volume, so a direct comparison settles it.
7	Air is a fluid and pushes upward on bodies in it, which is how hot air and helium balloons rise	The principle of upthrust applies to gases too, only the density of air is small.
8	0.8 N	Upthrust is real weight minus apparent weight, 5 minus 4.2 gives 0.8 N.
9	The same force acts on a tiny area, so the pressure at the tip is far larger than under the finger	Pressure is inversely proportional to area, a smaller area gives a larger pressure.
10	The mouth lowers the air pressure inside the straw and the atmospheric pressure outside pushes the water up	Atmospheric pressure drives the water, not any pulling force from the mouth.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

In pressure questions separate force in newtons from pressure in pascals and check whether the area is divided; in floating questions compare the density of the object with that of the liquid, or find upthrust from the weight of displaced fluid.Tutor Registration →

Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.