S3 Science: Pressure, Upthrust And Floating (Paper 1 (basics))

S3 Science: Pressure, Upthrust and Floating (Paper 1 (basics))

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 1 (basics))

This paper starts from the definition of pressure and practises force over area, the effect of a small contact area, pressure increasing with depth in a liquid, upthrust from displaced liquid, density as a test of floating, and the link between apparent weight and upthrust.

Direct answers　Pressure is force over area; a smaller area gives a larger pressure; deeper liquid gives greater pressure; upthrust equals the weight of displaced liquid; a density greater than the liquid means sinking; apparent weight is real weight minus upthrust.

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)

FoundationA plank pushes on a table with a force of 20 N over a contact area of 4 square metres. What is the pressure?
FoundationThe same 100 N force acts on an area of 0.5 square metres. What is the pressure?
FoundationA force of 5 N is applied through a needle tip of area 0.00001 square metres and a finger of area 0.0025 square metres. How many times larger is the pressure at the tip?
IntermediateWhy is the base of a dam built thicker than the top?
IntermediateAn object immersed in water displaces 200 cubic centimetres of water. What upthrust acts on it? (take g as 10 N/kg)
IntermediateA stone has a density of 2.5 g/cm³. When it is placed in water, does it float or sink?
ChallengeA metal block weighs 5 N in air and the spring balance reads 4 N when it is fully immersed in water. Find the upthrust and the volume of water displaced.
ChallengeA block of wood of density 0.6 g/cm³ floats on water. What fraction of its volume is above the surface?
ChallengeA diver is at 5 m and then at 15 m depth. What water pressure acts at each depth, and how many times larger is the deeper one? (water density 1000 kg/m³, g = 10 N/kg)
Error redoA student says the upthrust always equals the weight of the object. Is the statement correct?

03　Answers and one-line notes

#	Answer	One-line note
1	5 Pa	Pressure is force divided by area, 20 divided by 4 gives 5 pascals.
2	200 Pa	100 divided by 0.5 gives 200 pascals, a smaller area gives a larger pressure.
3	250 times: 500000 Pa at the tip against 2000 Pa at the finger	The same force over an area 250 times smaller gives a pressure 250 times larger.
4	Because pressure in a liquid increases with depth and the base carries the greatest water pressure.	Deeper water carries more weight above it, so the pressure is greater at the base.
5	2 N	The displaced water has a mass of 0.2 kg and a weight of 2 N, and upthrust equals the weight of displaced liquid.
6	It sinks, because its density is greater than that of water	Water has a density of about 1 g/cm³, and an object of greater density sinks.
7	Upthrust 1 N, displacing 100 cubic centimetres of water	Upthrust is the drop in reading, and 1 N of water corresponds to 0.1 kg or 100 cubic centimetres.
8	40 per cent of the volume	The submerged part is 0.6 of the volume, so the part above water is 0.4, which is 40 per cent.
9	50000 Pa at 5 m and 150000 Pa at 15 m, three times larger	Water pressure equals density times g times depth, so tripling the depth triples the pressure.
10	No: upthrust equals weight only when the object floats or is suspended, while a sinking object has upthrust smaller than its weight	Upthrust is set by the weight of displaced liquid and need not equal the weight of the object.

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.