Two identical bags, same weight — but narrow straps hurt Pawan's shoulders, while Megha's broad straps don't. Why does the same weight feel so different?
Key idea
Pressure = Force ÷ Area
The same weight (force) feels different depending on how much area it's spread across.
Worked example
A force of 100 N spread over 2 m² gives a pressure of 100 ÷ 2 = 50 N/m². The SI unit of pressure is N/m², also called the pascal (Pa).
Key idea
Liquids Exert Pressure
Height mattersAs a liquid column's height increases, so does the pressure at the bottom — that's why overhead water tanks are placed high, to boost tap pressure.
A dam's shape makes sense nowA dam's base is built broader than its top, because water pressure is greatest near the bottom.
Key idea
Liquids Press in Every Direction
Sideways too. Liquids press on the sides of a container, not only the bottom — shown by water spurting sideways from holes made at the same height around a bottle.
Key idea
Atmospheric Pressure
1
The atmosphere
the envelope of air around Earth — exerts pressure on everything, all the time.
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2
We aren't crushed because...
The pressure inside our bodies balances the pressure of the air outside. A rubber sucker sticks to a surface for the same reason — pushing out the air beneath it lowers the pressure inside.
Key idea
How Wind Forms
Wind. Air always moves from a region of high pressure to a region of low pressure. The greater the pressure difference, the higher the wind speed.
Key idea
Sea Breeze and Land Breeze
1
Daytime: sea breeze
Land heats faster than water. Warm air rises over the land, creating low pressure — so wind blows from sea to land.
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2
Night: land breeze
Land cools faster than the sea. Now the (relatively warmer) sea has low pressure — so wind blows from land to sea.
Key idea
Fast Wind Means Lower Pressure
High-speed winds are accompanied by reduced air pressure above the moving air.
Why open windows in a storm
Strong wind over a roof creates low pressure above it. If the pressure difference is large, a weak roof can blow off — keeping doors and windows open reduces this difference.
Key idea
How a Thunderstorm Forms
Warm moist air rises
Cools, forms clouds & rain
Strong winds separate charge
Charge builds up, then discharges
Key idea
Lightning and Thunder
1
Lightning
Positive charge (lighter ice) collects at the top of a cloud, negative charge (heavier droplets) lower down. When the charge builds up enough, it discharges suddenly as a bright flash.
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2
Thunder
Lightning rapidly heats the surrounding air, making it expand suddenly — producing the loud sound of thunder.
Key idea
Lightning Safety
1
During a storm
Stay away from tall objects, crouch in a low, open area, don't lie flat, avoid metal-rod umbrellas, and get out of water. A car is comparatively safe.
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2
Lightning conductor
A pointed metal rod on a building, connected to the ground, gives lightning a safe path to discharge — protecting the building.
Key idea
How a Cyclone Forms
Over warm ocean water, rising warm moist air lowers pressure further as its water vapour condenses and releases heat.
Earth's rotation adds the spin
Surrounding air rushes in and rises too, and Earth's rotation makes the whole system spin — creating a cyclone.
Key idea
The Eye of the Cyclone
Strong winds & heavy rain
Key idea
Cyclone Effects, and Watching the Sky
1
Destruction
Storm surges 3–12 m high, flooding, contaminated water and farmland, blocked roads, and power outages. Cyclones weaken once they move over land.
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2
Tracking and warning
The India Meteorological Department (IMD) tracks weather and issues cyclone alerts. Satellites help predict cyclone paths to reduce their impact.
Chapter · Key terms to remember
Key Terms
Pressure
Force per unit area; SI unit N/m² (pascal).
Atmospheric pressure
The pressure exerted by air around us.
Wind
Air moving from high to low pressure.
Chapter · Key terms to remember
More Key Terms
Sea breeze / Land breeze
Sea-to-land wind by day / land-to-sea by night.
Thunderstorm
A storm with lightning and thunder.
Lightning conductor
A rod giving lightning a safe path to ground.
Chapter · Key terms to remember
More Key Terms
Cyclone
A spinning storm system over warm ocean water.
Eye of the cyclone
The calm, lowest-pressure centre.
Questions for your notebook
Write these down, then discuss
1
True or False: (i) Air flows from higher to lower pressure. (ii) Liquids exert pressure only at the bottom of a container. (iii) The weather is stormy at the eye of a cyclone. (iv) It's safer to be in a car during a thunderstorm.
2
A boy lying flat on loose sand sinks less than the same boy standing upright on it. Why?
3
An elephant stands on four feet, each covering 0.25 m². If it weighs 20,000 N, what pressure does it exert on the ground?
Project as-is — students copy the questions, then the class discusses answers together.