HKDSE Physics | Pressure and Fluids Prep Guide (for tutors)

HKDSE Physics | Pressure and Fluids Prep Guide (for tutors) | P = F/A, Hydraulic Press, Archimedes

2026-11-01

Tutor lesson-prep guides · Secondary Physics

Students can say pressure equals force over area, but the hydraulic press looks like free force. This lesson brings conservation of energy in: force rises fifty times, so the piston moves fifty times less.

Objectives — Students can: (1) compute pressure with P = F/A and handle units; (2) use Pascal’s principle in a hydraulic press and explain the energy limit; (3) compute pressure with depth using rho g h and upthrust using rho V g, then decide floating or sinking.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Pressure unit	What one pascal equals	one newton per square metre
Density	Density in terms of mass and volume	rho = m/V
Equilibrium	Resultant force on a floating body	zero

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Defining pressure	P = F/A, smaller area means larger pressure	compare three cases
10-26 min	Hydraulic press	Pascal’s principle and force multiplication	get 10000 N
26-38 min	Pressure with depth	rho g h, independent of container shape	get 30 kPa
38-50 min	Upthrust	rho V g and floating conditions	get 20 N
50-56 min	Answer format	formula, substitution, unit, direction	rewrite one answer
56-60 min	Exit check	three questions marked on the spot	done unaided

03 Teaching script (read it out)

Scene 1: Pressure: the same force, different area

P = F/A. 600 N over 0.02 m2 gives 30000 Pa, or 30 kPa. Halve the area and pressure doubles, which is why blades are sharpened and snowshoes are wide. Always state the unit in pascals and check that cm2 has been converted: 1 cm2 is 10^-4 m2.

Task: the same 600 N on 0.005 m2 stiletto area (120 kPa).

Scene 2: Hydraulic press: force multiplies, work does not

Pascal’s principle: pressure is the same throughout an enclosed liquid, so F1/A1 = F2/A2. 200 N over 0.001 m2 gives 200000 Pa; acting on 0.05 m2 that yields 10000 N, a factor of 50. Volume is conserved, so the small piston travels fifty times further and the work done matches.

Task: explain why a 50-fold force gain is not free energy.

Scene 3: Pressure with depth and upthrust

Liquid pressure depends only on depth: P = rho g h = 1000 x 10 x 3 = 30000 Pa, regardless of how wide the container is. Upthrust equals the weight of displaced liquid: rho V g = 1000 x 0.002 x 10 = 20 N. Floating means upthrust equals weight; resting on the bottom means upthrust is smaller, and the submerged volume changes.

Reminder: once fully submerged, upthrust stays constant as depth increases.

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	Pressure	P = F/A with area in m2	30000 Pa
Calculation	Press output force	F2 = F1 x A2 / A1	10000 N
Calculation	Pressure with depth	P = rho g h	30 kPa
Judgement	Floating or sinking	compare upthrust with weight	floating means equal

Sanity check (teach this): Pressures should be realistic: a standing person is tens of kPa and each 10 m of water adds about 100 kPa. A press output must exceed its input when A2 exceeds A1, so a smaller result means the areas were swapped. Upthrust can never exceed the weight of displaced liquid.

05 Independent practice (3 questions)

Question	Answer	What to watch
600 N over 0.02 m2	30000 Pa	area is already in m2
Press with 200 N input and a 1:50 area ratio	10000 N	F2 = F1 x 50
Pressure at 3 m depth in water	30 kPa	rho g h, gauge pressure

06 Common mistakes and what to say

Mistake	Student writes	Say this
Leaving area in cm2	substituting 20 as the area	“1 cm2 is 10^-4 m2, convert it first.”
Swapping the piston areas	getting 4 N of output	“The big force acts on the big piston, so do not invert the areas.”
Using total water volume for depth pressure	claiming more water means more pressure	“Only depth matters, not the width.”
Using weight as upthrust on the bottom	treating a sunken block as floating	“Sunken means upthrust is less than weight.”

07 Homework (paper, section and question numbers)

Secondary Physics chapter index — all chapters
HKDSE Physics practice paper — Section A mechanics and fluids

08 Exit check (last 5 minutes)

Question	Answer	Decision
600 N over 0.02 m2	30000 Pa	three correct moves on to thermal physics
1:50 press with 200 N input	10000 N	one wrong means redo Pascal
Pressure at 3 m depth	30 kPa	two wrong means repeat act three

09 Tutor FAQ

Does a hydraulic press break energy conservation?

Force rises by the area ratio and displacement falls by the same ratio, so F times d is unchanged.

Is atmospheric pressure included?

Questions asking for liquid pressure want rho g h only; total pressure adds the atmosphere.

Does upthrust change with depth?

Only through displaced volume, so a fully submerged object feels the same upthrust at any depth.

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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.