In our daily life, we apply force in many ways. We push doors, carry school bags, hammer nails into walls, and cut vegetables with knives. The effect of force depends not only on its magnitude but also on the area over which it acts. This gives rise to the concept of pressure. Pressure is the force acting normally (perpendicularly) on a unit area of a surface. In simple words, pressure tells us how concentrated a force is on a particular area.
Pressure = Force ÷ Area Where P = Pressure, F = Force and A = Area. The SI unit of pressure is Pascal (Pa). 1 Pascal = 1 Newton of force acting on 1 square metre area.
Pressure is inversely proportional to area. This means smaller area creates Greater pressure while Larger area creates smaller pressure.
Some examples of concept of pressure
A sharp knife has a very small cutting edge. The same force acts on a smaller area, producing a large pressure that cuts objects easily.
Broad straps increase the contact area on the shoulders. Since pressure decreases when area increases, the bag feels more comfortable to carry.
The pointed tip has a very small area. Therefore, even a small force produces high pressure, allowing the nail to penetrate wood or walls easily.
Pressure in the liquids can be understood by the fluid pressure. Liquids also exert pressure because they have weight. Every layer of liquid presses on the layers below it. As a result, pressure exists at every point inside a liquid.
The deeper we go in a liquid, the greater the pressure. This happens because the weight of the liquid above increases with depth.
Unlike solids, liquids exert pressure not only downward but also sideways and upward. This can be observed when water leaks out from holes made in the sides of a container.
A taller column of liquid exerts greater pressure at its base. This principle is used in water supply systems.
Atmosphere: The Earth is surrounded by a thick layer of gases called the atmosphere. The atmosphere consists mainly of Nitrogen (78%), Oxygen (21%) and Carbon dioxide and other gases (1%). These gases have weight and therefore exert pressure. The pressure exerted by the weight of the air surrounding the Earth is called atmospheric pressure. Atmospheric pressure acts on all objects from every direction.
Atmospheric pressure on our bodies is enormous. However, our body fluids and gases exert an equal pressure outward. Since these pressures balance each other, we do not feel crushed.
Wind is the movement of air from one place to another. Wind is caused mainly by differences in air pressure.
Air always moves from a region of high pressure to a region of low pressure. This movement continues until the pressure becomes equal.
The Sun heats different parts of the Earth unevenly.
When air becomes warm:
This creates a low-pressure region. Cooler and heavier air from nearby high-pressure regions moves in to occupy the vacant space. This movement of air is called wind.
These are local winds caused by unequal heating of land and sea. Sea Breeze Occurs During Daytime. During the day, the land heats faster than sea water. Air over land becomes warm and rises. A low-pressure area develops over land causing cooler air from the sea moves toward land. This movement of air is called sea breeze. Sea breeze cools coastal regions during hot afternoons. Land Breeze occurs During Night. At night the land cools faster than water therefore water remains relatively warmer. Air above the sea rises causing cooler air from land moves toward the sea. This movement of air is called land breeze. Land breeze helps maintain temperature balance between land and sea.
HIGH-SPEED WINDS AND AIR PRESSURE
Air pressure and wind speed are closely related. Scientists have observed that when air moves very fast, the pressure exerted by that air decreases. This principle explains many natural phenomena, including the lifting of roofs during storms and the movement of aircraft.
Principle of wind speed
Higher Wind Speed → Lower Air Pressure. Lower Wind Speed → Higher Air Pressure. This means that rapidly moving air creates a region of lower pressure.
Balloon Experiment
Procedure
Observation
The balloons move towards each other instead of moving apart.
Explanation
This demonstrates shows that high-speed air lowers pressure.
Roofs Blown Off During Storms
During strong storms, high-speed winds pass over roofs. Pressure above the roof decreases while pressure inside the house remains relatively high. This pressure difference creates an upward force on the roof. As a result, weak roofs may be lifted or blown away.
Safety Measure: Keeping doors and windows slightly open allows pressure to equalize and reduces the chances of roof damage.
STORMS
A storm is a violent weather condition characterized by strong winds and often accompanied by rain, thunder, lightning, or hail. Storms occur when large pressure differences develop in the atmosphere.
Conditions Required for Storm Formation
Importance of Storms
Though destructive, storms help to distribute heat across the Earth. It maintain atmospheric balance and bring rainfall to many regions.
THUNDERSTORMS
A thunderstorm is a storm accompanied by thunder, lightning, heavy rain, and strong winds. Thunderstorms are common in tropical countries such as India, especially during summer.
Formation of Thunderstorms
Step 1: Heating of Earth's Surface- The Sun heats the land strongly.
Step 2: Warm Air Rises- Warm moist air becomes lighter and rises rapidly.
Step 3: Cooling- As the air rises, it cools.
Step 4: Condensation- Water vapour condenses into tiny droplets.
Step 5: Cloud Formation- Large towering clouds called cumulonimbus clouds develop.
Step 6: Strong Air Currents- Powerful upward and downward air currents form inside the clouds.
Step 7: Charge Formation- Collisions between ice particles and water droplets create electric charges.
Step 8: Lightning and Thunder-Electrical discharge occurs, producing lightning and thunder.
Characteristics of Thunderstorms
Lightning is a sudden discharge of electricity in the atmosphere. It is one of the most powerful natural electrical phenomena. Temperatures during lightning may reach about 30,000°C, which is hotter than the surface of the Sun.
Formation of Lightning
Charge Separation
Inside thunderclouds:
As charges continue to accumulate, the difference between them becomes very large.
Electrical Discharge
When the charge difference becomes strong enough to overcome the insulating property of air, electricity suddenly flows. This sudden flow of charge produces a bright flash called lightning.
Types of Lightning
THUNDER
The loud sound produced by lightning is called thunder. Lightning heats the surrounding air extremely rapidly.
As a result:
Why Is Lightning Seen Before Thunder Is Heard?
Light travels much faster than sound. Speed of Light is ≈ 300,000 km/s while the speed of Sound is
≈ 340 m/s. Therefore, we see the flash first and hear thunder later.
SAFETY MEASURES DURING LIGHTNING
Lightning can cause severe injuries and loss of life. Therefore, safety precautions are extremely important. Some safety precautions are;
Don'ts during lightening
A cyclone is a huge rotating storm system that develops around a low-pressure centre and is accompanied by strong winds, heavy rainfall, and thunderstorms. Cyclones usually form over warm tropical oceans.
Formation of Cyclones
Step 1: Heating of Ocean Water- The Sun heats the ocean surface. Warm water evaporates rapidly.
Step 2: Warm Air Rises- The warm moist air above the ocean rises. This creates a low-pressure region.
Step 3: Condensation- As the air rises higher, it cools. Water vapour condenses into clouds.
Step 4: Release of Heat-Condensation releases latent heat. This heat warms the surrounding air. The air rises even faster.
Step 5: More Air Rushes In- More air moves toward the low-pressure centre. The cycle continues and intensifies.
Step 6: Rotation-Due to the Earth's rotation, incoming air begins to spiral. A rotating storm system develops.
Step 7: Cyclone Formation
The system grows into a powerful cyclone with:
Structure of a Cyclone
Effects of Cyclones
Cyclone Preparedness
Before a Cyclone
During a Cyclone
After a Cyclone
Role of Technology in Cyclone Warning
Modern technology helps in:
Important Tools
Early warnings have greatly reduced loss of life during cyclones.
Different Situations
Many objects around us demonstrate this principle of gravity: