Junior Science: Pressure and Upthrust Tutor Prep Guide | Solid Pressure, Liquid Pressure and Floating

2026-11-14

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Students hear pressure and think of stress, and they believe floating objects have the biggest upthrust. Both are wrong. This chapter separates solid pressure from liquid pressure with two formulas, then turns floating into a calculation of displaced volume so that nobody has to guess.

Objectives — Students can: (1) calculate the pressure a block exerts when resting on faces of different area and explain the difference; (2) compare liquid pressure at two depths using density times gravity times depth, and state that the shape of the container does not matter; (3) decide whether an object floats, sinks or stays suspended by comparing upthrust with weight, and work out the fraction that sits above the surface.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Force and weight	What is the weight of a one kilogram object?	About ten newtons
Area	What is the area of a face measuring 0.2 metres by 0.2 metres?	0.04 square metres
Density	What is the density of water?	One thousand kilograms per cubic metre

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-6 min	One block, two faces	The same block lying flat and standing on its side	Predict which face presses harder
6-20 min	Pressure in solids	Force over area: 40 N gives 1000 and 2000 pascals	Calculate the side-on pressure
20-34 min	Pressure in liquids	Density times gravity times depth: 0.5 m gives 5000 pascals	Calculate the pressure at 2 m
34-48 min	Upthrust and floating	Two litres displaced gives 20 N of upthrust	Decide whether a 24 N block floats
48-56 min	Applications	Knife edge, snowshoe, diver, iceberg	Say whether area or density was changed
56-60 min	Exit check	Collect three questions and mark at once	Complete independently

03 Teaching script (read it out)

Scene 1: Solid pressure: the same force, a different area

Separate two words that everyday speech runs together. Force is measured in newtons; pressure is the force on each square metre, measured in pascals. There is one formula: pressure equals force divided by area. A block weighing 40 newtons on a face of 0.04 square metres gives 1000 pascals; the same block standing on a face of 0.02 square metres gives 2000 pascals. Once those numbers are on the board the intuition catches up: the force did not change at all, the area halved, so the pressure doubled. Let students classify examples themselves. A sharpened knife reduces area. A snowshoe and a tank track increase it. A bed of nails supports a person because each nail carries a small share of the weight. Warn about units: square centimetres must be divided by ten thousand before use, and that single slip moves the answer by four orders of magnitude.

Students do: write one sentence for each of three examples saying which area changed and how the pressure responded.

Scene 2: Liquid pressure: depth and density only

Liquid pressure is density times gravity times depth, and all three quantities must be in standard units: kilograms per cubic metre, ten metres per second squared, and metres. Two values anchor the formula: at a depth of 0.5 metres the pressure is 5000 pascals, and at 2 metres it is 20000 pascals, so quadrupling the depth quadruples the pressure. The misconception that must be killed is shape. At the same depth, a narrow vessel, a wide vessel and a bent tube all give the same pressure, because the weight of liquid above and the area it acts on grow together. Use atmospheric pressure for one striking number: one atmosphere is about one hundred thousand pascals and supports a water column about ten metres high, which is why a suction pump cannot lift water beyond roughly ten metres. Link it to something students have felt: ears hurt when diving and pop when a plane climbs, and both are pressure differences, one in water and one in air.

Students do: compare the pressure at the same depth in three differently shaped containers and state which quantity is shared.

Scene 3: Upthrust: let the displaced volume do the talking

Upthrust comes from a pressure difference: the bottom of an object is deeper than the top, so the upward pressure is larger and the difference produces a net upward force. The calculable form is Archimedes: upthrust equals liquid density times gravity times displaced volume. A block of two litres fully submerged displaces 0.002 cubic metres, so the upthrust is twenty newtons. Floating or sinking is then only a comparison. A weight of 24 newtons beats an upthrust of 20 newtons, leaving a net 4 newtons downwards, so it sinks. A weight of exactly 20 newtons can stay suspended at any depth. For a floating body use the fraction: ice at about nine hundred kilograms per cubic metre in water at one thousand has ninety per cent of its volume submerged, so roughly a tenth sits above, which is exactly why icebergs are dangerous. Correct one sentence that comes up every year: upthrust does not exist only for floating things. A steel block on the bottom still has upthrust, its weight is simply larger.

Students do: take three blocks of different weight and write the net force direction and the outcome for each.

04 Question types and worked steps

Type	Example	Steps	Answer
Pressure calculation	Find the pressure of a 40 N block standing on its side	Identify the area first, then divide, then write pascals	Inserting square centimetres without converting
Liquid pressure comparison	Compare 0.5 m and 2 m in the same liquid	Only the depth changes, so the ratio matches the ratio of depths	Treating container shape as a factor
Upthrust calculation	Find the upthrust on a two litre object fully submerged	Density of liquid times gravity times displaced volume	Using the object density instead of the liquid density
Floating judgement	Decide the outcome for 24 N weight against 20 N upthrust	Compare the two forces and state the net direction	Repeating heavy things sink without comparing numbers

Sanity check (teach this): Sanity check: only three formulas are used, force over area, density times gravity times depth, and density times gravity times displaced volume, with no derivation of hydraulic principles; solid and liquid pressure occupy separate acts because merging them is the commonest error at this level; floating is decided by comparing numbers rather than by memorising a slogan; the diver and iceberg examples can be pictured directly; atmospheric pressure appears only as one striking figure rather than a particle model.

05 Independent practice (3 questions)

Question	Answer	What to watch
Pressure	Calculate the pressure of one block lying flat and standing on its side	Check that areas were converted to square metres
Liquid pressure	Calculate 0.5 m and 2 m, then state the ratio	Check that depth rather than volume was used
Floating	Decide the outcome for three blocks of different weight	Check that upthrust was calculated before comparing

06 Common mistakes and what to say

Mistake	Student writes	Say this
Force and pressure merged	Saying a smaller area gives a bigger force	The force is unchanged, the pressure is bigger. Say which quantity you mean; they do not share a unit.
Square centimetres used directly	Substituting 20 square centimetres into the formula	The formula needs square metres. Divide 20 by ten thousand first; that step is missing.
Object density in the upthrust formula	Using the density of steel for a steel block in water	The upthrust formula uses the liquid density. For steel in water you still use water.
No upthrust when sunk	Saying a sunken object has no upthrust	Its weight is simply larger than the upthrust. Write both forces out and you will see it.

07 Homework (paper, section and question numbers)

Junior Science chapter index — All chapters for this subject
Tutor prep guide index — Matching chapters in other subjects
Redo this chapter — Practice questions 1, 2 and 4: pressure, liquid pressure, floating

08 Exit check (last 5 minutes)

Question	Answer	Decision
Solid pressure	Force over area in pascals	Square centimetres means the first act again
Liquid pressure	Density times gravity times depth, shape irrelevant	Tying it to volume means the second act again
Floating	Compare upthrust with weight and give the net direction	No upthrust when sunk means the third act again

09 Tutor FAQ

Must all three formulas be memorised?

Yes, but only three, and each one with its units, because most lost marks come from units rather than formulas.

Why skip hydraulic machines?

They are an extension topic. Juniors need the relationships between pressure, area and depth far more than a second machine.

Can floating be answered with a slogan?

A slogan survives simple questions and fails on the fraction-above-water question, so this chapter always calculates upthrust first.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.