Junior Physics: Pressure and Buoyancy Tutor Prep Guide | Pressure, Liquid Depth and Three Floating Cases

2026-11-12

Tutor lesson-prep guides · Junior Secondary Physics

Students recite that pressure equals force over area, then explain a sharp knife by saying thin cuts better, never that a smaller area raises pressure. Buoyancy is messier: many believe heavy objects must sink. This lesson uses three judgements – area, depth, then upthrust against weight – to join both halves into one chain.

Objectives — Students can: (1) explain knife edges, bag straps and drawing pins using pressure equals force over area, and predict how pressure changes when the area changes; (2) state that liquid pressure depends only on density and depth, not on the shape or the total amount of liquid, and calculate it; (3) find upthrust from the weight of displaced liquid and use upthrust against weight to decide floating, suspending or sinking.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Unit of force	What is a newton the unit of?	Force; weight is also a force
Density	How is density calculated?	Mass divided by volume
Area conversion	How many square metres is one square centimetre?	One ten-thousandth, that is 0.0001

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-6 min	Two opening questions	Same book lying flat and standing on its edge	Write an answer now, compare at the end
6-20 min	Solid pressure	Force over area; halve the area and pressure doubles	Compute knife edge and bag strap pressure
20-34 min	Liquid pressure	Density and depth only, shape is irrelevant	Calculate the pressure at two metres depth
34-48 min	Where upthrust comes from	Pressure difference on top and bottom, weight of displaced liquid	Calculate the upthrust on a stone
48-56 min	Three floating cases	Upthrust greater, equal or smaller than weight	Classify three objects
56-60 min	Exit check	Collect two questions, mark at once	Complete independently

03 Teaching script (read it out)

Scene 1: Solid pressure: the area is the denominator

The definition is one sentence: pressure equals force divided by the area carrying it. Students remember it and then forget the denominator in application. Use the same 600 N force twice: on 0.02 square metres the pressure is 30000 Pa, that is 30 kPa. Halve the area and the pressure doubles, which explains a sharpened blade, a pointed pin and a wide bag strap in one sentence each. Insist on units: newtons over square metres give pascals, while square centimetres inflate the answer ten thousand times, the classic way calculation questions die.

Students do: compute the strap pressure before and after making it wider, and say which feels better.

Scene 2: Liquid pressure: depth and density only

Liquid pressure depends only on density and depth, so the formula is one product: density times gravitational field strength times depth. Water at two metres gives 20000 Pa, that is 20 kPa, whether the container is a cup or a tank. This is the counter-intuitive part: a tank and a narrow tube at the same depth give the same pressure, which is best shown with two containers side by side. Depth is measured from the free surface, not from the bottom, and students routinely misread a sloping container. Finish with the dam wall: the base is thicker because pressure grows with depth.

Students do: draw three differently shaped containers at the same depth and state which has the greatest pressure.

Scene 3: Upthrust and floating: compute upthrust, then compare with weight

Upthrust comes from the pressure difference between the bottom and the top, and its size equals the weight of the displaced liquid, which is a computable quantity. Example: a stone weighs 6 N and displaces 200 cubic centimetres of water, which is 0.2 kg, so the upthrust is 2 N. Floating needs one comparison: if upthrust exceeds weight the object rises and floats, if they are equal it stays suspended, if upthrust is smaller it sinks. Here 2 N against 6 N leaves 4 N downwards, so the stone sinks. Clear two misconceptions: heavy objects do not always sink, since a steel ship floats by displacing a large volume, and light objects do not always float, since a coin sinks. Close with the floating condition: when an object floats, upthrust equals its weight, so knowing the weight tells you how much water it displaces.

Students do: classify three objects by comparing upthrust with weight and write the reason for each.

04 Question types and worked steps

Type	Example	Steps	Answer
Solid pressure calculation	Find pressure from force and area	Convert to square metres, divide, answer in pascal or kilopascal	Use the actual contact area
Liquid pressure calculation	Find pressure at a given depth	Take depth from the surface, multiply density, g and depth	Never use the total height of the container
Upthrust calculation	Find upthrust from displaced volume	Convert volume to cubic metres, multiply by density, then by g	State whether the liquid or the gas is displaced
Floating decision	Decide whether an object floats, suspends or sinks	Compute upthrust, compute weight, compare	When floating, upthrust equals weight, not the largest possible value

Sanity check (teach this): Sanity check: the lesson uses two formulas and one comparison – area for solids, depth for liquids, upthrust against weight for floating; every example is verifiable on the spot (600 N over 0.02 square metres is 30000 Pa; 200 cubic centimetres of water weighs 2 N), so arithmetic errors surface immediately; all areas and volumes are converted to base units first to avoid the ten-thousand-fold error; every question must state which quantity is the denominator or where depth is measured, and those two habits block most lost marks.

05 Independent practice (3 questions)

Question	Answer	What to watch
Pressure comparison	Two pressures from one force and two areas	Check that areas are in square metres
Liquid pressure	Two containers of different shape at the same depth	Check that the answer is the same value
Floating decision	Classify three objects	Check that upthrust is computed before comparing

06 Common mistakes and what to say

Mistake	Student writes	Say this
Forgetting to convert area	Substituting square centimetres into the formula	The denominator must be square metres, so divide square centimetres by ten thousand first.
Judging liquid pressure by amount	Assuming more water means more pressure	At the same depth a cup and a tank give the same pressure. Look at depth, not at how much water you have.
Assuming heavy objects sink	Treating a floating steel ship as an exception	Floating compares upthrust with weight; a ship displaces enough water for upthrust to win.
Using the largest upthrust when floating	Writing that a floating object has upthrust greater than weight	A floating object has upthrust equal to weight. If upthrust were greater it would keep rising instead of settling at the surface.

07 Homework (paper, section and question numbers)

Junior physics chapter index — All chapters for the subject
Prep guide master index — Similar chapters in other subjects
Redo this chapter — Practice questions 2, 4 and 6: pressure conversion, liquid pressure, floating decision

08 Exit check (last 5 minutes)

Question	Answer	Decision
Pressure formula	Force over area in square metres	A wrong unit means replay scene two
Liquid pressure	Density and depth only	Answering shape or amount means replay scene three
Floating decision	Upthrust against weight	Judging by heaviness alone means replay scene four

09 Tutor FAQ

Must students learn the wording of Archimedes principle?

No. Remember that upthrust equals the weight of displaced liquid; every calculation follows from that one sentence.

Does atmospheric pressure belong here?

It sits in the same unit but is a separate lesson, so this guide keeps to solids and liquids and leaves gases for their own session.

Floating or suspended?

Floating means part of the object is above the surface with upthrust equal to weight; suspending means it is fully submerged and still, also with upthrust equal to weight.

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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.