CLASS 8 SCIENCE · CHAPTER 6 · STUDY NOTES
Pressure, Winds, Storms, and Cyclones
Understand pressure, winds, storms, and cyclones through short explanations, labelled models and original practice. Read one topic, explain it in your own words, then check.
EXPLORE
The same force can press more or less
Use contact area to explain pressure.Same force, different contact area
- Pressure
- Force acting perpendicular to a surface divided by the area over which it acts.
- Pascal (Pa)
- One newton per square metre: 1 Pa = 1 N/m².
For the same force, a smaller area gives greater pressure. Broad bag straps spread the force over more of the shoulder; a sharp tip concentrates it.
Check both units before calculating. Do not divide a force in N by an area in cm² and label the answer Pa without converting the area.
Force per unit area
P = F ÷ A = 120 ÷ 3 = 40 PaDivide the perpendicular force by the area in m².
Take a moment. What do you think?
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Question
A 120 N force presses on 3 m². What pressure does it produce?
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EXPLORE
Deeper water gives greater pressure
Explain why depth matters more than container width.Follow the depth below the surface
- Liquid pressure
- Pressure exerted by a liquid on surfaces in contact with it.
Liquids exert pressure in all directions. In the same liquid at rest, greater depth below the surface gives greater pressure. At the same depth, pressure does not depend on how wide the container is.
An elevated tank gives a greater water-column height above a lower tap. Dam walls need to withstand greater pressure near the bottom.
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Question
Two containers hold the same liquid to the same height. One is wider. Is its bottom pressure necessarily greater?
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EXPLORE
Air pushes on surfaces too
Explain a suction cup through a pressure difference.Outside pressure holds the cup
- Atmospheric pressure
- Pressure exerted by the air around us.
Pressing a suction cup against a smooth surface reduces the air pressure beneath it. Higher outside pressure helps hold it against the surface.
Air inside an inflated balloon pushes outward in all directions. Atmospheric pressure also acts outside; material tension and the pressure difference determine its shape.
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Question
Why may a suction cup fail to stick to a rough wall?
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EXPLORE
Air moves because pressure differs
Explain the direction of a daytime sea breeze.A daytime sea breeze
- Wind
- Moving air.
- Pressure difference
- A difference in air pressure between regions that can drive air movement.
In a simple daytime model, land heats faster than nearby sea. Warm air rises over land, and cooler surface air moves from sea towards land. At night the direction can reverse.
This explains a local breeze; real weather also involves Earth’s rotation, terrain and larger weather systems. Two rubber balloons are not a universal model of all atmospheric motion.
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Question
In the daytime sea-breeze model, does surface air move from land to sea or sea to land?
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EXPLORE
A pressure difference can lift a roof
Explain why forces on two sides of a surface need comparing.Compare above and below
- Net force
- The combined effect of forces acting on an object.
Fast airflow over a roof can lower pressure above it relative to the inside in a simple model. A large pressure difference can produce an upward force on the roof.
Real wind damage also depends on gusts, building design and debris. This model is an explanation, not an instruction to open windows during a storm. Follow official warnings and shelter guidance.
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Question
If pressure below a roof is greater than above it, which direction is the resulting pressure force?
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EXPLORE
Lightning is a sudden electrical discharge
Connect rising moist air, charge separation and thunder.See the difference between charge and discharge
- Thunderstorm
- A storm accompanied by lightning and thunder.
- Lightning
- A sudden electrical discharge within clouds, between clouds or between cloud and ground.
Rising moist air cools and water vapour condenses into cloud droplets. Collisions among ice particles and droplets can separate charge. A large electric field can let a sudden discharge pass through air.
Lightning heats air rapidly. The rapid expansion produces thunder. A lightning conductor provides a designed path to the ground; installation needs qualified expertise.
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Question
Why are lightning and thunder connected?
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EXPLORE
A cyclone draws energy from warm seas
Explain the roles of moisture, pressure and Earth’s rotation.Look at the centre and the surrounding storm
- Cyclone
- A large rotating storm system around a low-pressure centre.
- Storm surge
- A rise in coastal water level caused by a storm, above the expected tide.
Warm moist air rises over the sea. Condensation releases heat, helping rising air continue. Surrounding air moves towards lower pressure, and Earth’s rotation helps produce a spinning system.
The eye may be relatively calm, but dangerous winds surround it. Heavy rain, storm surge and flooding can cause damage even when wind strength falls after landfall.
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Question
Why does a cyclone usually weaken after moving well inland?
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EXPLORE
Warnings help people act early
Choose appropriate shelter and follow official advice.Warning → decision → shelter
- Forecast
- A prediction based on observations and a scientific model.
IMD uses observations, satellites and other tools to monitor storms and issue warnings. Follow local authorities, prepare essentials and use designated shelters when instructed.
When thunder is heard, move promptly into a substantial enclosed building or an enclosed metal-topped vehicle. An isolated tree or an open shelter is not safe. Crouching outdoors does not make lightning exposure safe.
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Question
The eye of a cyclone passes over your area and the wind becomes calm. Does that mean the danger is over?
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