Pressure: force per unit area
Two friends carry equally heavy school bags. One bag has thin cord straps that dig into the shoulders; the other has wide padded straps and feels comfortable. The weight is the same, so something else must be different.
Pressure is the force acting on each unit of area. Pressure = force ÷ area. Its SI unit is newton per square metre (N/m²), also called the pascal (Pa).
Key idea 1: Area decides the pressure — For the same force, a small area gives a large pressure and a large area gives a small pressure. In this chapter we count only the force that acts perpendicular (at right angles) to the surface.
- Easy: A force of 60 N on an area of 3 m² gives a pressure of 20 Pa.
- Medium: If the area is doubled for the same force, the pressure becomes half.
- Exam-type: A box of weight 200 N rests on a face of 0.5 m². The pressure is 200 ÷ 0.5 = 400 Pa; on a face of 1 m² it is only 200 Pa.
In the figure: The same push gives a high pressure on a small area and a low pressure on a large area.
Pressure: force per unit area
In the app you can move this figure with your finger.
Open in the appKey idea 2: Using pressure in daily life — We make the area small to get a high pressure: a pointed nail, a sharp knife. We make the area large to get a low pressure: wide bag straps, a round pad under a pot on the head, wide tractor tyres, camel feet on sand.
- Easy: A sharp blade cuts well because its edge has a tiny area.
- Medium: A camel does not sink in soft sand because its broad feet spread the weight over a large area.
- Exam-type: A girl of weight 400 N on shoes of total area 0.04 m² presses with 10000 Pa; on skis of 0.4 m² the pressure is only 1000 Pa.
Pressure = force ÷ area; the unit is the pascal (N/m²).
Thinking that a heavier object always gives a greater pressure. The area matters too.
Look at the area first. Memory clue: Small area = Sharp pressure; Large area = Light pressure.
Spot the difference
| Area | Pressure | Example |
|---|---|---|
| small | high | nail tip |
| large | low | wide bag strap |
Practice questions
- 1What is the SI unit of pressure?
- 2Why does a bag with wide straps feel more comfortable?
- 3Which cuts better, a sharp knife or a blunt knife? Why?
- 4A force of 90 N acts on an area of 3 m². Find the pressure.
- 5A girl of weight 400 N stands on two shoes, each of area 0.02 m². Find the pressure on the floor.
- 6Why do tractors have very wide rear tyres?
- 7A box of weight 200 N has faces of 1 m² and 0.5 m². Find the pressure when it rests on each face. On which face is it greater?
- 8A person of weight 600 N wears flat sandals of total area 0.04 m². Another person of weight 500 N wears high heels of total area 0.002 m². Who presses the floor harder?
Show answers (try them yourself first)
- 1Newton per square metre (N/m²) or pascal (Pa).
- 2The weight is spread over a larger area, so the pressure is less.
- 3A sharp knife, because its thin edge has a small area and gives a high pressure.
- 490 ÷ 3 = 30 Pa.
- 5Area = 0.04 m²; pressure = 400 ÷ 0.04 = 10000 Pa.
- 6They spread the weight over a large area, so the pressure is low and the tractor does not sink.
- 7200 ÷ 1 = 200 Pa and 200 ÷ 0.5 = 400 Pa; the pressure is greater on the 0.5 m² face.
- 8First: 600 ÷ 0.04 = 15000 Pa; second: 500 ÷ 0.002 = 250000 Pa; the person in high heels presses harder.
Pressure exerted by liquids
In a three-storey building the water tank is on the roof. The tap on the ground floor gives a strong stream, while the tap on the top floor only dribbles. The same tank gives different streams at different floors.
A liquid presses on the bottom and on the side walls of its container, that is, in all directions. The pressure grows with the height of the liquid column above the point.
Key idea 1: Deeper means more pressure — Make three holes at different heights in a bottle full of water. The lowest hole shoots the water farthest because the water column above it is the tallest. In an overhead tank, a higher tank gives more pressure at the taps.
- Easy: Water squirts sideways from a hole in a bottle, so liquids press on side walls.
- Medium: The ground-floor tap is farthest below the tank, so its stream is the strongest.
- Exam-type: Two tanks, one narrow and one wide, with water at the same height of 3 m give the same pressure at their bases.
In the figure: The deeper the hole, the farther the jet goes, because the pressure is larger there.
Key idea 2: Where it matters — The base of a dam is made much broader than its top because the sideways water pressure is greatest near the bottom. Leaking pipe joints spurt water because water presses on the pipe walls. Divers and submarines feel more pressure when they go deeper.
- Easy: A dam is thicker at the bottom than at the top.
- Medium: A diver going deeper feels an increasing water pressure.
- Exam-type: To get a stronger stream from a tank, raise its height; making the tank wider does not help.
Liquid pressure depends on the height (depth) of the liquid column, not on the width of the vessel.
Thinking that a larger tank with more water gives more pressure. Only the height of the water level matters.
Think of the water above you. Memory clue: Deep = Dense push; Shallow = Slight push.
Spot the difference
| Depth | Pressure | Water jet |
|---|---|---|
| shallow | less | weak |
| deep | more | strong |
Practice questions
- 1In which directions does a liquid exert pressure?
- 2Why are water tanks placed on rooftops?
- 3Why is the base of a dam made broader?
- 4A bottle has holes at 5 cm, 10 cm and 15 cm above its base and is full of water. Which jet goes farthest?
- 5Does a wider tank give a stronger stream than a narrow tank at the same height? Give a reason.
- 6What happens to the pressure on a diver as she goes deeper?
- 7In a three-storey building, which floor's tap gives the strongest stream, if the tank is on the roof? Explain.
- 8A balloon is tied over the bottom of a pipe. How does its bulge change as water is poured up to 30 cm and then 60 cm?
Show answers (try them yourself first)
- 1In all directions: on the bottom and on the sides.
- 2A high water column gives more pressure at the taps.
- 3The sideways water pressure is greatest near the bottom, so the wall must be broad there.
- 4The lowest hole (5 cm), where the water column above is tallest.
- 5No; the pressure depends only on the height of the water, which is the same.
- 6It increases, because the water column above her gets taller.
- 7The ground floor, because it is farthest below the tank and has the tallest water column.
- 8The bulge becomes larger at 60 cm, because the taller water column gives more pressure.
Atmospheric pressure
A rubber suction hook sticks firmly to a smooth bathroom tile and holds a heavy towel. There is no glue on it, yet it hangs there. Something invisible pushes it onto the wall.
The envelope of air around the Earth is the atmosphere. It contains nitrogen, oxygen, argon, carbon dioxide and other gases. The pressure that this air exerts on everything is the atmospheric pressure.
Key idea 1: Air presses on everything — Air presses on all objects in all directions. When you press a rubber sucker on a smooth surface, most of the air inside is pushed out, so the pressure inside is low; the higher outside air pressure holds it on. The atmosphere presses with a force of about 2250 N on an area of 15 cm × 15 cm, about the weight of 225 kg.
- Easy: A balloon swells in all directions because the air inside presses on its walls everywhere.
- Medium: A paper plate covered with an unfolded chart paper is harder to lift than one covered with a folded sheet, because the air force grows with area.
- Exam-type: We are not crushed because the pressure inside our body equals the atmospheric pressure.
In the figure: The sucker sticks because the pressure outside is higher than the pressure inside.
Key idea 2: Units and flow of air — The practical unit of air pressure is the millibar (mb), equal to the hectopascal (hPa). 1 hPa = 100 Pa, so 1000 hPa is 100000 Pa. Air always moves from a region of higher pressure to a region of lower pressure; that is why air rushes out of a punctured tube.
- Easy: Weather maps give air pressure in hPa or mb.
- Medium: When a bicycle tube gets a puncture, the air flows from the higher pressure inside to the lower pressure outside.
- Exam-type: A sucker on a rough surface does not stick well because air leaks in and the pressure difference cannot be kept.
1 hPa = 1 mb = 100 Pa; air presses on every object.
Thinking that air has no weight or no pressure. Air presses with a very large force all around us.
Compare inside and outside pressures. Memory clue: Equal = Easy (no crushing, no sticking); Different = Pushed (the higher side pushes).
Spot the difference
| Case | Inside pressure | Outside pressure | Result |
|---|---|---|---|
| Our body | equal to outside | atmospheric | not crushed |
| Suction cup | low | higher | sticks to the wall |
Practice questions
- 1What is the atmosphere?
- 2Name the practical unit of air pressure.
- 3In which direction does air flow, from high pressure or from low pressure?
- 4Why does a suction hook stick to a smooth tile?
- 5Why are we not crushed by the atmospheric pressure?
- 6Convert 1000 hPa into pascals.
- 7A suction hook falls suddenly when a tiny hole is made in it. Explain.
- 8The atmosphere presses on an area of 15 cm × 15 cm with about 2250 N. What mass has a weight of 2250 N, taking 10 N for every kg?
Show answers (try them yourself first)
- 1The envelope of air around the Earth.
- 2The millibar (mb) or hectopascal (hPa).
- 3From high pressure to low pressure.
- 4The air under it is pushed out, so the higher outside pressure presses it onto the tile.
- 5The pressure inside our body is equal to the atmospheric pressure, so the two balance.
- 61000 × 100 = 100000 Pa.
- 7Air enters through the hole and the inside pressure becomes equal to the outside pressure, so nothing holds the hook.
- 82250 ÷ 10 = 225 kg.
Formation of wind and low pressure in fast winds
On a hot afternoon at the beach, a cool breeze blows from the sea towards the land. By midnight the breeze reverses and blows from the land towards the sea. Something changes in the air pressure over land and sea.
Wind is moving air. It blows from a region of high air pressure to a region of low air pressure, and it blows faster when the pressure difference is larger. Fast-moving air has a lower pressure.
Key idea 1: Wind comes from a pressure difference — Join an inflated balloon and a flat balloon with a straw: air flows from the inflated one to the flat one until the pressures become equal. In the day the land heats faster, its warm light air rises and a low pressure forms, so the sea breeze blows in. At night the sea is warmer, so the land breeze blows out to sea.
- Easy: Air moves from high pressure to low pressure.
- Medium: Air flow stops when both regions reach equal pressure.
- Exam-type: In the afternoon the wind blows from the sea to the land, so the land has the lower pressure.
In the figure: The wind always blows towards the place with the lower pressure.
Key idea 2: Fast air means low pressure — If you blow between two hanging balloons, they move together, because the pressure between them falls and the higher outside pressure pushes them in. In the same way, a strong wind over a weak roof lowers the pressure above it and the higher pressure below can blow the roof away. So it is safer to keep doors and windows open in a storm, and banners and hoardings have holes to let the wind through.
- Easy: A strip of paper rises when you blow across its top.
- Medium: Open doors and windows reduce the difference between the pressure inside the house and above the roof.
- Exam-type: Holes in a banner let the wind pass through, so a smaller force acts on it.
In the figure: Fast air between the balloons has a low pressure, so the outer higher pressure pushes them together.
Wind blows from high pressure to low pressure; fast air has low pressure.
Thinking that a smaller pressure difference gives a faster wind. A larger difference gives a faster wind.
Follow the low pressure. Memory clue: Wind goes to Low; fast air has Low pressure (Low-Low).
Spot the difference
| Time | Hotter place | Wind direction |
|---|---|---|
| day | land | sea to land (sea breeze) |
| night | sea | land to sea (land breeze) |
Practice questions
- 1What is wind?
- 2At what time does a sea breeze blow?
- 3What happens to air pressure where air moves fast?
- 4Why does the land have a low pressure in the afternoon?
- 5Why is it safer to keep the doors and windows open in a storm?
- 6Why do the two balloons move towards each other when air is blown between them?
- 7Balloon P has a higher pressure than balloon Q and they are joined by a straw. What happens to their sizes, and when does the flow stop?
- 8Why are small holes made in big hoardings?
Show answers (try them yourself first)
- 1Moving air.
- 2During the day, from the sea towards the land.
- 3The pressure falls.
- 4The land heats faster, so the warm air above it rises and leaves a low pressure.
- 5The pressure inside and above the roof become nearly equal, so the roof is not pushed off.
- 6The pressure between them falls and the higher outside pressure pushes them together.
- 7P shrinks and Q grows; the flow stops when the pressures become equal.
- 8The wind passes through the holes, so a smaller force acts on the hoarding.
Thunderstorms, lightning and cyclones
On a humid afternoon in the rainy season, dark clouds gather, the wind rises, lightning flashes and a loud thunder follows. Before the main monsoon, such storms also bring the mango showers that help the fruit to ripen.
Strong winds with rain make a storm. A storm with lightning and thunder is a thunderstorm. A cyclone is a huge spinning storm that forms over warm ocean water, with a calm eye at its low-pressure centre.
Key idea 1: Lightning and thunder — Warm moist air rises, cools and forms clouds. Strong winds moving up and down rub ice particles and water droplets and charge them: lighter ice particles with positive charge collect at the top and heavier water droplets with negative charge at the bottom. Air is an insulator, but when the charges grow very large it breaks down and a sudden flow of charge makes a flash, the lightning. It heats the air so quickly that the air expands and makes thunder.
- Easy: A storm with lightning and thunder is a thunderstorm.
- Medium: During lightning, crouch in a low open place; do not lie flat, avoid tall trees and metal-rod umbrellas.
- Exam-type: A lightning conductor is a metal rod with a pointed end above the roof and the other end buried deep; it carries the charge safely into the ground.
In the figure: The cloud top is positive, the bottom is negative, and the ground below becomes positive.
Key idea 2: Cyclones — Over a warm ocean the moist air rises, its water vapour condenses and gives out heat, so the air rises still more and the pressure becomes lower. Air rushes in and, because the Earth rotates, it spins. The centre, the eye, is calm with the lowest pressure; around it are very strong winds and heavy rain. Over land the cyclone loses its moist air supply and weakens. Storm surges, floods, landslides and salty soil are its damages; the IMD and satellites warn us in advance, and we should go to cyclone shelters.
- Easy: A cyclone forms over warm ocean water.
- Medium: The weather is calm at the eye of a cyclone.
- Exam-type: A cyclone weakens over land because its supply of warm, moist ocean air is cut off.
In the figure: The calm eye is surrounded by a ring of very strong winds and heavy rain.
Cloud charges lead to lightning; lightning heats the air and makes thunder; cyclones form over warm oceans.
Thinking that the eye of a cyclone is stormy. The eye is calm; the ring around it is the most violent part.
Think of the order of events. Memory clue: Rub, Charge, Flash, Bang (rubbing, charging, lightning, thunder).
Spot the difference
| Part of a cyclone | Weather |
|---|---|
| eye | calm, lowest pressure |
| ring around the eye | very strong winds, heavy rain |
Practice questions
- 1What is a thunderstorm?
- 2Where do cyclones form?
- 3What is the centre of a cyclone called?
- 4What makes thunder?
- 5Which part of a thundercloud is positively charged?
- 6Why does a cyclone weaken after reaching land?
- 7Would big lightning flashes occur if air were a very good conductor? Explain.
- 8Name two dangers of a cyclone and two things we can do to stay safe.
Show answers (try them yourself first)
- 1A storm with lightning and thunder.
- 2Over warm ocean water.
- 3The eye.
- 4Lightning heats the air so quickly that the air expands suddenly and makes a loud sound.
- 5The upper part, where the lighter ice particles collect.
- 6Its supply of warm, moist ocean air is cut off.
- 7No; the charges would leak away gradually before a large amount could build up.
- 8Dangers: storm surge and floods (also landslides, salty soil). Safety: follow IMD warnings and move to a cyclone shelter.