ScienceCBSE Class 8 · Pressure, Winds, Storms, and Cyclones

Pressure in Solids, Liquids and Air: Class 8 Science

Updated 17 October 2026 · Fact-checked

Pressure is the force acting perpendicular to a surface per unit area: pressure = force ÷ area, in newton per square metre (N/m2), called pascal (Pa). A smaller area gives a larger pressure for the same force. Liquids exert pressure on the bottom and sides in all directions, increasing with the height of the liquid column, and the air around us exerts atmospheric pressure.

Understand Pressure: Force per Unit Area, Liquid Pressure and Atmospheric Pressure

Pressure. Two bags of equal weight hurt the shoulder differently: the one with narrow straps spreads its weight over a smaller area. Pressure is the force per unit area:

Pressure = Force ÷ Area.

Here we take only forces perpendicular to the surface. The SI unit of force is the newton and of area the square metre, so the SI unit of pressure is N/m2, called the pascal (Pa). Example from the chapter: a force of 100 N on a cardboard of area 2 m2 gives 100 ÷ 2 = 50 N/m2.

Same force, different area. When the area is smaller, the pressure is higher. That is why a nail is driven by its pointed end and an apple is cut with the sharp edge of a knife. Broad straps, broad bucket handles and a round cloth pad under a load on the head reduce pressure by increasing the area.

Pressure of liquids. In Activity 6.1 two pipes of different diameters but with equal heights of water show equal bulging of balloons even though the weight of water differs. So the bulge depends on the height of the water column, not on the weight or width; more height gives a larger bulge. Hence the pressure of a liquid in a vessel depends on the height of its column, and overhead tanks are placed high so that the pressure at the taps is greater. In Activity 6.2 water spurts out of side holes in a bottle: liquids exert pressure on the sides as well as on the bottom, in all directions. The base of a dam is broader because the horizontal pressure is very large near the bottom.

Pressure of air. The envelope of air round the Earth is the atmosphere (nitrogen, oxygen, argon, carbon dioxide and others). In Activity 6.3 an inverted paper plate covered by an unfolded sheet is harder to lift than one covered by a folded sheet: the weight of the sheet is unchanged, but air presses on a larger area. Air exerts pressure on all objects: atmospheric pressure. A blown balloon expands in all directions because the air inside presses in all directions. A rubber sucker sticks because air pressure outside is higher than the reduced pressure inside. The force from the atmosphere on 15 cm x 15 cm is nearly equal to the weight of a 225 kg object (2250 N). We are not crushed because the pressure inside our bodies equals the atmospheric pressure.

Other units. The practical unit of air pressure is the millibar (mb), 1 mb = 100 Pa; the hectopascal (hPa) is also 100 Pa. In the cyclone figure the pressure falls to 994 mb at the centre against 1008 mb outside.

The next topic covers how pressure differences make wind, storms and cyclones.

Pressure: Force per Unit Area, Liquid Pressure and Atmospheric Pressure: formulas and key facts

Pressure: $P = F / A$; Force perpendicular to the surface; F in N, A in m2, P in N/m2 or Pa.
Force from pressure: $F = P \times A$; Rearranged form, to find the force on a given area.
Area from force and pressure: $A = F / P$; Convert cm2 to m2 first: 1 m2 = 10 000 cm2.
Unit of pressure: $1 P a = 1 N / m 2$; Also 1 mb = 1 hPa = 100 Pa for air pressure.
Liquid pressure depends on: Height of the liquid column; Not on the width of the vessel (Activity 6.1). Acts in all directions.

How to solve Pressure: Force per Unit Area, Liquid Pressure and Atmospheric Pressure questions

Use this method for pressure numericals and reasoning.

1List force (N) and area (m2). Convert cm2 to m2 (divide by 10 000) and mb to Pa (multiply by 100).
2Write P = F / A and substitute with units.
3Calculate carefully and write the unit as N/m2 or Pa.
4For comparisons, keep the force the same and compare areas: smaller area, greater pressure.
5For liquids, compare heights of the liquid column only.
6Check the answer with a sensible estimate (halving the area doubles the pressure).

Pressure: Force per Unit Area, Liquid Pressure and Atmospheric Pressure 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.

Narrow versus broad strap

Suppose a school bag weighs 60 N. Narrow straps touch the shoulder over 0.002 m2 in all, and broad straps over 0.006 m2 in all (invented values).

(a) Find the pressure with each type of strap. (b) How many times is the narrow-strap pressure greater? (c) Why do people place a round cloth under a head load?

Try it first, then show the solutionSolution

Narrow: 60 ÷ 0.002 = 30 000 Pa. Broad: 60 ÷ 0.006 = 10 000 Pa.
30 000 ÷ 10 000 = 3 times.
The cloth spreads the weight over a larger area and reduces the pressure.

Answer: (a) 30 000 Pa and 10 000 Pa; (b) 3 times; (c) larger area, less pressure.

Memory hook: Spread the weight, soften the pressure.

Overhead tank

Imagine an overhead tank on the roof of a three-storey building supplies taps on all floors.

(a) Which floor's tap gets the stronger stream, and why? (b) Why is the tank not placed on the ground? (c) A wider tank of the same height is installed; does the pressure at the bottom change?

Try it first, then show the solutionSolution

The lowest floor: it is farthest below the water level, so the water column above it is highest and the pressure is greatest.
Placed at a height, the tank provides larger column height and so higher pressure at the taps.
No: liquid pressure depends on the height of the column, not on the width of the vessel (Activity 6.1).

Answer: (a) Lowest floor; (b) to increase pressure at the taps; (c) no change if the height of water is the same.

Memory hook: Taller column, stronger stream.

Tricks to remember Pressure: Force per Unit Area, Liquid Pressure and Atmospheric Pressure

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

P = F over A
Pressure sits on top of nothing: Force on the top, Area at the bottom; small area makes big pressure.
Use it when: Recalling the formula and its meaning.
Sharp is small area
Pointed nail, sharp knife: tiny area, big pressure.
Use it when: Why sharp objects cut.
Height not width
Liquid pressure listens to height only.
Use it when: Liquid pressure questions.

Pressure: Force per Unit Area, Liquid Pressure and Atmospheric Pressure: solved examples

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

Example 1: A simple pressure numerical · 2 marks · Short answerBoard level

A force of 240 N acts perpendicular to a surface of area 0.8 m2. Find the pressure.

Try it first, then show the solutionSolution

P = F / A = 240 N ÷ 0.8 m2. (1)
P = 300 N/m2 = 300 Pa. (1)

Answer: 300 Pa

Example 2: Why nails are pointed · 1 mark · MCQBoard level

A nail is easier to drive in by its pointed end because
(a) the force becomes larger
(b) the area becomes smaller so the pressure becomes larger
(c) the area becomes larger
(d) the nail becomes lighter

Try it first, then show the solutionSolution

The same force acts on a tiny area at the point, giving high pressure.

Answer: (b)

Example 3: Brick pressure on three faces · 5 marks · Long answerHarder board level

A brick weighs 30 N and has faces of 20 cm x 10 cm, 20 cm x 5 cm and 10 cm x 5 cm.
(a) Convert each area to m2. (1)
(b) Find the pressure when the brick rests on each face. (3)
(c) Which face gives the greatest pressure and by how many times is it greater than the least? (1)

Try it first, then show the solutionSolution

(a) 200 cm2 = 0.02 m2; 100 cm2 = 0.01 m2; 50 cm2 = 0.005 m2. (1)
(b) 30 ÷ 0.02 = 1500 Pa; 30 ÷ 0.01 = 3000 Pa; 30 ÷ 0.005 = 6000 Pa. (3)
(c) The 10 cm x 5 cm face gives 6000 Pa; 6000 ÷ 1500 = 4 times the least. (1)

Answer: (b) 1500 Pa, 3000 Pa, 6000 Pa; (c) smallest face, 4 times.

Example 4: Two pipes with balloons · 4 marks · Case-basedHarder board level

Read the passage and answer.

Two transparent pipes of the same length but different diameters are filled with water to the same height; each has a balloon over its lower end. Then more water is added to the narrower pipe.
(a) How do the bulges of the balloons compare at the start? (1)
(b) Why, although the weights of water differ? (1)
(c) What happens after adding water to the narrow pipe? (1)
(d) How does this explain the position of overhead tanks? (1)

Try it first, then show the solutionSolution

(a) The bulges are equal. (1)
(b) Liquid pressure depends on the height of the water column, not on its weight or the width of the pipe. (1)
(c) The balloon of the narrow pipe bulges more because the height increased. (1)
(d) A tank at a height gives a taller water column to the taps, so the pressure and the stream are higher. (1)

Answer: (a) Equal; (b) height decides; (c) bulges more; (d) height increases pressure at the taps.

Example 5: Atmospheric pressure puzzles · 4 marks · Case-basedHOTS / competency

Read the passage and answer.

A rubber sucker pressed on a smooth wall is hard to pull off. A student says: 'The sucker is glued to the wall.' Another says: 'We should be crushed by the air on us.'
(a) Explain why the sucker sticks. (1)
(b) How much force does the atmosphere exert on a 15 cm x 15 cm area according to the chapter? (1)
(c) Explain why we are not crushed. (1)
(d) How is the force in (b) consistent with a pressure in Pa? Find it using area 0.15 m x 0.15 m and 2250 N. (1)

Try it first, then show the solutionSolution

(a) Pressing pushes out air, so the pressure inside is reduced; the higher air pressure outside presses it against the wall. (1)
(b) About 2250 N, equal to the force of gravity on 225 kg. (1)
(c) The pressure inside our bodies equals the atmospheric pressure and balances it. (1)
(d) Area = 0.15 × 0.15 = 0.0225 m2; P = 2250 ÷ 0.0225 = 100 000 Pa (about 1000 mb). (1)

Answer: (a) Lower pressure inside; (b) 2250 N; (c) internal pressure balances it; (d) 100 000 Pa.

Where students lose marks in Pressure: Force per Unit Area, Liquid Pressure and Atmospheric Pressure

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

Mistake: Using cm2 without conversion.
Areas on small objects are in cm2.
Wrong working
P = 60 N ÷ 20 cm2 = 3 Pa
Right working
20 cm2 = 0.002 m2; P = 60 ÷ 0.002 = 30 000 Pa
Do this: Divide cm2 by 10 000 to get m2.
Usually costs: 1 to 2 marks.
Mistake: Saying liquid pressure depends on the width of the vessel.
A wide vessel looks heavier.
Wrong working
A wider pipe gives greater pressure at the bottom.
Right working
Pressure depends on the height of the liquid column.
Do this: Remember Activity 6.1.
Usually costs: 1 mark.
Mistake: Mixing force and pressure.
Both seem like pushes.
Wrong working
Pressure is measured in newton.
Right working
Pressure is measured in N/m2 (pascal); newton is the unit of force.
Do this: Write the unit with every answer.
Usually costs: ½ to 1 mark.
Mistake: Saying liquids press only downwards.
Gravity acts downwards.
Wrong working
Water exerts pressure only on the bottom.
Right working
Water exerts pressure on the bottom and on the sides, in all directions.
Do this: Recall the spurting holes.
Usually costs: 1 mark.

Exam tips

Write the formula, substitution and unit in every numerical.
Convert area to m2 before dividing.
Explain 'sharp' and 'broad' in terms of area, with the force unchanged.
Quote activities by what they show: height decides liquid pressure; air presses on a larger area.
Mention that atmospheric pressure is balanced by the pressure inside our bodies.

Practice questions on Pressure: Force per Unit Area, Liquid Pressure and Atmospheric Pressure

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

Q1 · 2 marks · Very short answerBoard level
Define pressure and state its SI unit with its special name.
Hint
Force per unit area.
Answer
Pressure is force per unit area (1); SI unit is N/m2 called pascal (Pa) (1).
Q2 · 1 mark · MCQBoard level
Water spurts out sideways from holes in a bottle. This shows that liquids
(a) exert pressure only downwards
(b) exert pressure on the walls of the container
(c) have no pressure
(d) exert pressure only upwards
Hint
Activity 6.2.
Answer
(b)
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The base of a dam is made broader than the top.
Reason (R): Water pressure acts horizontally and is very large near the bottom.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Connect the broad base to pressure at the bottom.
Answer
(a) R explains why the base is broader.
Q4 · 4 marks · Short answerHarder board level
A rectangular tank of base 1.5 m x 0.8 m holds water that weighs 4800 N. (a) Find the base area. (b) Find the pressure on the base due to this weight. (c) If the same tank is placed on a smaller stand of area 0.4 m2, find the new pressure. (d) How many times has the pressure changed?
Hint
Use P = F / A in each case.
Answer
(a) 1.5 × 0.8 = 1.2 m2. (b) 4800 ÷ 1.2 = 4000 Pa. (c) 4800 ÷ 0.4 = 12 000 Pa. (d) 12 000 ÷ 4000 = 3 times (1 mark each).
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
A girl of weight 450 N stands on both feet, each foot having an area of 150 cm2 in contact with the ground, and then on one foot. A boy of weight 600 N stands on both his feet of 200 cm2 each.
(a) Find the girl's pressure on both feet in Pa. (b) Find her pressure on one foot. (c) Find the boy's pressure on both feet. (d) Who exerts the greater pressure on both feet?
Hint
150 cm2 = 0.015 m2; 200 cm2 = 0.02 m2.
Answer
(a) Area = 0.03 m2; P = 450 ÷ 0.03 = 15 000 Pa. (b) 450 ÷ 0.015 = 30 000 Pa. (c) Area = 0.04 m2; P = 600 ÷ 0.04 = 15 000 Pa. (d) Both are equal at 15 000 Pa (1 mark each).

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.What is the formula for pressure?Show answer
Pressure = force ÷ area.
2.What is 1 pascal?Show answer
1 N/m2.
3.What does liquid pressure depend on?Show answer
The height of the liquid column.
4.Why is the base of a dam broad?Show answer
To withstand the large horizontal water pressure near the bottom.
5.How many pascal is 1 millibar?Show answer
100 Pa.

Pressure: Force per Unit Area, Liquid Pressure and Atmospheric Pressure: frequently asked questions

Why do we not feel the atmospheric pressure?

The pressure inside our bodies equals the atmospheric pressure and balances it.

Why does a sucker stick to a wall?

Pressing it removes most of the air inside, so the pressure inside is lower than the air pressure outside.

Is the weight of the water responsible for the pressure at the bottom?

In Activity 6.1 pipes of different widths show equal bulges at the same height, so the height of the column is what decides it.

Why do porters place a cloth pad on the head?

The pad increases the area on which the weight acts and reduces the pressure.