PhysicsICSE Class 8 · Force and Pressure

Pressure: P = F/A, Pressure in Liquids and Atmospheric Pressure (ICSE Class 8 Physics)

Updated 17 October 2026 · Fact-checked

Pressure is the force acting normally on unit area: P = F/A, in pascal (1 Pa = 1 N/m²). The same force gives more pressure on a smaller area. Liquid pressure rises with depth; the atmosphere presses with about 100 000 Pa at sea level.

Understand Pressure in Solids, Liquids and the Atmosphere

Pressure. The force acting normally (at right angles) on unit area of a surface:

$P = \frac{F}{A}$

SI unit: pascal (Pa), where $1 Pa = 1 N/m^{2}$ . Another unit: N/cm², with $1 N/cm^{2} = 10000 Pa$ . Rearranged: $F = P \times A$ .

Same force, different area.

Smaller area, larger pressure: knives, needles, nails, drawing pins, high heels.
Larger area, smaller pressure: wide school-bag straps, broad tyres, camel feet, snow shoes.

Changing units. $1 m^{2} = 10000 cm^{2}$ , so $1 cm^{2} = 0.0001 m^{2}$ . Convert the area to m² before dividing, so that the answer is directly in pascal. The force on a body resting on a table is its weight, $m g$ with $g = 10$ m/s².

Pressure in liquids (qualitative):

a liquid presses on the bottom and the walls of its container;
pressure increases with depth: water from the lowest hole in a bottle spurts farthest;
at the same depth, the pressure is the same in all directions;
it depends on depth (and the liquid), not on the shape of the container.

Applications: dams are thicker at the base; deep-sea divers wear strong suits; a tap on the ground floor runs with more force than one on a top floor.

Atmospheric pressure. Air has weight, so the atmosphere presses on everything. At sea level it is about $100000$ Pa ( $1 \times 1 0^{5}$ Pa), about 100 000 N on every square metre. We are not crushed because the pressure inside our bodies balances it, and air presses equally from all sides.

Evidence: a straw (lower the pressure inside, outside air pushes the drink up), a syringe or dropper, a can crushed when the air is removed from inside, a rubber sucker on a wall.

Pressure in Solids, Liquids and the Atmosphere: formulas and key facts

Pressure: $P = \frac{F}{A}$; Pa = N/m²; F normal to the surface.
Force from pressure: $F = P \times A$; Convert A to m².
Unit links: $1 N/cm^{2} = 10000 Pa 1 cm^{2} = 0.0001 m^{2}$; Atmospheric pressure ≈ 10⁵ Pa.
Weight on a surface: $F = m g$; g = 10 m/s².

How to solve Pressure in Solids, Liquids and the Atmosphere questions

Find the force and the area, convert the area to m², then divide.

1On a table or floor, the force is the weight $m g$ (mass in kg, $g = 10$ ).
2Convert areas: cm² × 0.0001 = m²; N/cm² × 10 000 = Pa.
3Use $P = \frac{F}{A}$ ; for several feet or tyres, multiply the area by the number.
4For liquids, compare depths: greater depth, greater pressure.
5Remember atmospheric pressure ≈ 100 000 Pa at sea level.

Pressure in Solids, Liquids and the Atmosphere in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Heels and tractors

Suppose a tractor of mass 6000 kg has four tyres each in contact with 0.3 m² of ground, and a woman of mass 50 kg stands on two heels, each of area 2 cm².

Find the pressure under each and compare them. ( $g = 10$ m/s²)

Try it first, then show the solutionSolution

Tractor: $F = 60000$ N; $A = 1.2$ m²; $P = 50000$ Pa.
Woman: $F = 500$ N; $A = 4 cm^{2} = 0.0004$ m²; $P = 1250000$ Pa.
Ratio $= \frac{1 250 000}{50 000} = 25$ .

Answer: 50 000 Pa and 1 250 000 Pa; the heels press 25 times harder.

Memory hook: Area matters more than weight.

Why the dam is thick at the bottom

Imagine a water tank with three small holes at 10 cm, 20 cm and 30 cm below the surface.

Which jet goes farthest and why?

Try it first, then show the solutionSolution

The lowest hole is deepest.
Pressure increases with depth, so water leaves it fastest and lands farthest.

Answer: The lowest jet, because the pressure is greatest at the greatest depth.

Memory hook: Deeper means harder.

Tricks to remember Pressure in Solids, Liquids and the Atmosphere

Short memory aids. Say each one aloud once, then use it on a question the same day.

P = F over A
"Pressure is force Per Area": small area, big pressure.
Use it when: Formula and logic.
10 000 trick
N/cm² × 10 000 = Pa, because 1 m² has 10 000 cm².
Use it when: Unit change.
Straw story
Pressing out the air lets outside pressure push the drink up.
Use it when: Atmospheric pressure.

Pressure in Solids, Liquids and the Atmosphere: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Force from pressure · 1 mark · MCQBoard level

A pressure of 400 Pa acts normally on a surface of area 0.25 m². The total force on the surface is:
(a) 1600 N
(b) 100 N
(c) 400 N
(d) 0.0006 N

Try it first, then show the solutionSolution

$F = P \times A = 400 \times 0.25 = 100$ N.

Answer: (b) 100 N

Example 2: Unit change · 2 marks · Short answerBoard level

Convert a pressure of 5 N/cm² into pascal.

Try it first, then show the solutionSolution

$1 N/cm^{2} = 10000$ Pa. (1)
$5 \times 10000 = 50000$ Pa. (1)

Answer: 50 000 Pa

Example 3: Brick on three faces · 4 marks · Long answerHarder board level

A brick of weight 24 N measures 20 cm × 10 cm × 5 cm. Find the pressure when it rests on (a) its largest face, (b) its smallest face. (c) Find the ratio.

Try it first, then show the solutionSolution

(a) $A = 20 \times 10 = 200 cm^{2} = 0.02$ m²; $P = \frac{24}{0.02} = 1200$ Pa. (1½)
(b) $A = 10 \times 5 = 50 cm^{2} = 0.005$ m²; $P = \frac{24}{0.005} = 4800$ Pa. (1½)
(c) $\frac{4800}{1200} = 4$ , which is the ratio of the areas $200 : 50$ . (1)

Answer: (a) 1200 Pa (b) 4800 Pa (c) 4 : 1

Example 4: Standing on one foot · 3 marks · Short answerHarder board level

A boy of mass 45 kg stands on both feet, each of area 150 cm², then on one foot. Find the pressure in each case. ( $g = 10$ m/s²)

Try it first, then show the solutionSolution

$F = 45 \times 10 = 450$ N. (½)
Both feet: $A = 300 cm^{2} = 0.03$ m²; $P = \frac{450}{0.03} = 15000$ Pa. (1)
One foot: $A = 0.015$ m²; $P = \frac{450}{0.015} = 30000$ Pa, twice as much. (1½)

Answer: 15 000 Pa on both feet; 30 000 Pa on one foot

Example 5: Why a nail goes in · 4 marks · Case-basedHOTS / competency

A hammer exerts 80 N on the head of a nail of area 4 cm². The point has an area of 0.01 cm². Assume the force reaches the point undiminished. (a) Find the pressure on the head. (b) Find the pressure at the point. (c) By what factor is the pressure raised? (d) State why the nail enters the wood but the head does not.

Try it first, then show the solutionSolution

(a) $A = 4 \times 1 0^{- 4}$ m²; $P = \frac{80}{4 \times 1 0 ^{- 4}} = 2 \times 1 0^{5}$ Pa. (1)
(b) $A = 1 0^{- 6}$ m²; $P = \frac{80}{1 0 ^{- 6}} = 8 \times 1 0^{7}$ Pa. (1)
(c) $\frac{8 \times 1 0 ^{7}}{2 \times 1 0 ^{5}} = 400$ . (1)
(d) The tiny area of the point gives a very high pressure that the wood cannot resist. (1)

Answer: (a) 2 × 10⁵ Pa (b) 8 × 10⁷ Pa (c) 400 times

Where students lose marks in Pressure in Solids, Liquids and the Atmosphere

The slips examiners see most often, and what to write instead.

Mistake: Not converting cm² to m²
Pascal needs m².
Wrong working
$\frac{24}{200} = 0.12$ Pa.
Right working
$\frac{24}{0.02} = 1200$ Pa.
Do this: Multiply cm² by 0.0001 first.
Mistake: Mass used as force
The force on the surface is the weight.
Wrong working
$P = \frac{45}{0.03}$ .
Right working
$P = \frac{450}{0.03}$ .
Do this: Weight = mass × 10.
Mistake: Counting one foot only
Standing on both feet doubles the area.
Wrong working
"Area = 150 cm² for the standing boy."
Right working
"Area = 300 cm² for both feet."
Do this: Count how many feet, tyres or heels touch the ground.

Exam tips

Write P = F/A and the units (Pa = N/m²).
Convert areas to m² before dividing.
In qualitative answers, give the reason: depth or area.
Use 100 000 Pa for atmospheric pressure unless a value is given.

Practice questions on Pressure in Solids, Liquids and the Atmosphere

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · Very short answerBoard level
In the same liquid, is the pressure greater at 2 m or at 5 m below the surface?
Hint
Think of depth.
Answer
At 5 m, because liquid pressure increases with depth.
Q2 · 2 marks · Short answerHarder board level
Take atmospheric pressure as 100 000 Pa. Find the force of the air on the front cover of a textbook of size 30 cm by 20 cm. Why can you lift the book easily?
Hint
$F = P \times A$ with the area in m².
Answer
Area $= 0.30 \times 0.20 = 0.06$ m²; $F = 100000 \times 0.06 = 6000$ N. The air presses equally on the top and the bottom, so the forces balance.
Q3 · 3 marks · Short answerHarder board level
A pressure of 25 000 Pa acts on a piston of area 0.04 m². Find the force. Then find the pressure if the same force acts on an area of 0.01 m².
Hint
Use $F = P \times A$ , then $P = \frac{F}{A}$ .
Answer
$F = 25000 \times 0.04 = 1000$ N. New pressure $= \frac{1000}{0.01} = 100000$ Pa.
Q4 · 3 marks · Short answerHarder board level
Explain with reasons why (a) a dam is built thicker at its base, (b) a camel can walk on sand, (c) a drinking straw works.
Hint
Depth, area, atmospheric pressure.
Answer
(a) Water pressure is greatest at the bottom, so the base must be strongest. (b) Wide feet spread the weight over a large area, giving low pressure. (c) Sucking lowers the air pressure inside the straw, and the higher atmospheric pressure outside pushes the liquid up.
Q5 · 4 marks · Long answerHOTS / competency
A 60 kg student wears shoes with two soles of area 200 cm² each. Another 60 kg student wears heels of area 4 cm² each on two heels. Compare the pressures when standing on both. ( $g = 10$ m/s²)
Hint
Convert cm² to m² for both.
Answer
Force $= 600$ N for both. Shoes: $A = 400 cm^{2} = 0.04$ m²; $P = 15000$ Pa. Heels: $A = 8 cm^{2} = 0.0008$ m²; $P = 750000$ Pa. The heels give $\frac{750 000}{15 000} = 50$ times the pressure, because the area is 50 times smaller.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.SI unit of pressure?Show answer
Pascal (N/m²).
2.1 N/cm² in Pa?Show answer
10 000 Pa.
3.Liquid pressure increases with…?Show answer
Depth.
4.Atmospheric pressure at sea level?Show answer
About 100 000 Pa.
5.Smaller area, same force: pressure?Show answer
Greater.

Pressure in Solids, Liquids and the Atmosphere: frequently asked questions

What is pressure?

Pressure is the force acting normally on unit area of a surface, P = F/A. Its SI unit is the pascal (Pa), equal to one newton per square metre.

Why does a sharp knife cut better than a blunt one?

A sharp edge has a very small area, so the same force produces a much higher pressure and cuts easily.

What is atmospheric pressure?

It is the pressure caused by the weight of the air around Earth. At sea level it is about 100 000 Pa.