Pressure, Winds, Storms, and Cyclones | Class 8 Science Notes

Pressure

Pressure is a fundamental concept that relates force and area. It is defined as the force exerted per unit area on a surface. This concept explains everyday observations such as why carrying a bag with broad straps feels more comfortable than with narrow straps, even if both bags weigh the same. The force due to the weight of the bag acts on the shoulder, but when the strap is narrow, the force is concentrated on a smaller area, resulting in higher pressure and discomfort. Conversely, broad straps distribute the force over a larger area, reducing the pressure and making it easier to carry the bag. This principle is expressed mathematically as Pressure = Force / Area, where the force is perpendicular to the surface.

The SI unit of force is newton (N), and the unit of area is square meter (m²). Therefore, the SI unit of pressure is newton per square meter (N/m²), also called pascal (Pa). For example, if a force of 100 N acts on an area of 2 m², the pressure exerted is 50 N/m² or 50 Pa.

This concept is applicable in various situations such as lifting water buckets with broad or narrow handles, where broad handles reduce pressure on the hands, making it easier to lift. Similarly, placing a cloth under loads carried on the head increases the area over which the force acts, reducing pressure and discomfort.

Pressure also explains why nails are easier to drive when struck on the pointed end rather than the flat head, and why sharp knives cut better than blunt ones. These examples show that a smaller area of force application results in higher pressure, facilitating cutting or piercing.

Liquids exert pressure as well, which depends on the height of the liquid column. This is why overhead water tanks are placed at a height—to increase water pressure at taps, ensuring a good flow. Liquids exert pressure in all directions, not just at the bottom, which can be observed by water spurting out from holes on the sides of a container.

In summary, pressure is a measure of how concentrated a force is on a surface, influencing many natural and practical phenomena.

📊 Diagram: Fig. 6.1 shows Megha and Pawan carrying bags with broad and narrow straps respectively, illustrating how strap width affects pressure on shoulders. Fig. 6.2 shows buckets with broad and narrow handles, demonstrating ease of lifting with broad handles. Fig. 6.3 depicts persons carrying loads on their heads with cloth pieces underneath to increase area and reduce pressure.

🧪 Activity: Activity 6.1 involves using two transparent pipes of different diameters with balloons attached, filled with water to the same height, to observe that pressure depends on the height of the water column, not the diameter.

🔗 Connection: This section lays the foundation for understanding pressure exerted by liquids and gases, leading to the study of atmospheric pressure and wind formation.