NCERT Class 8 Science Curiosity Chapter 06 Pressure Winds Storms and Cyclones PDF Download

Chapter 6: Pressure, Winds, Storms, and Cyclones

Probe and ponder

• Why are winds stronger on some days than on others?
• Why are water tanks usually placed at a height?
• Can air pressure really crush us?
• What causes storms and cyclones? If the Earth stopped rotating, would cyclones still form?
• Share your questions

You must have observed fallen leaves on the ground swirling in the air or being swept away, and trees swaying or even bending when a strong wind blows. Have you ever wondered why fallen leaves rise in the air or trees sway or bend? Does the wind exert force on fallen leaves to make them rise or on trees to bend? Recall other similar effects of the force exerted by wind like slamming of doors or rattling of windows, or fluttering of clothes? How does the force exerted by wind make this happen? The force exerted by wind creates wind pressure which causes these effects. In this chapter, we will explore the relationship between force and pressure, and understand how they shape powerful natural events like thunderstorms and cyclones.

6.1 Pressure

Megha and her brother Pawan are going on a picnic. They walk to the picnic spot, carrying identical items in their bags (Fig. 6.1). On the way, Pawan keeps adjusting his bag, and looks uncomfortable. Megha asks, "Is there a problem with your bag?" Pawan responds, "Yes, it is hurting my shoulders." Megha says, "Both our bags are equally heavy. Why does your bag hurt, and mine doesn't?" Pawan reflects for a minute and says, "Perhaps, it is because of the difference in the straps of our bags. My bag has narrow straps while your bag has broad straps."

Can the shape or size of the straps really make a difference? Let us try to find out.

When we carry a bag, we feel its weight because of the force of gravity acting on our shoulders. The weight of the bag with narrow straps acts on a smaller area of our shoulders, whereas the weight of the bag with broad straps is spread out over a larger area of our shoulders. It is due to this reason that we feel more comfortable carrying a bag with broader straps than one with narrow straps, although both bags have the same weight. Since the area over which the force acts is involved, we define a quantity called pressure, which is the force per unit area.

So, \[ \text{Pressure} = \frac{\text{Force}}{\text{Area}} \]

At this stage, we will consider only those forces which act perpendicular to the surface on which the pressure is to be computed.

[Figure 6.1: Megha and Pawan carrying their bags, See in your textbook]

Teacher's Note

The key insight is that the same force (the weight of the bag) creates different effects depending on the area. When the force is spread over a larger area, the pressure is smaller, so it feels more comfortable. Remember: smaller area means higher pressure, and larger area means lower pressure. Always use the formula \( \text{Pressure} = \frac{\text{Force}}{\text{Area}} \) carefully - if area doubles but force stays the same, pressure is cut in half.

Broad straps reduce the pressure exerted by the bag on our shoulders as compared to narrow straps. Therefore, we feel more comfortable carrying a bag with broad straps. Can you now understand why it feels easier to lift a water-filled bucket with a broad handle than with a narrow handle (Fig. 6.2)? Similarly, we have seen that when people carry loads like pots or vegetable baskets on their heads, they often place a round piece of cloth under the loads (Fig. 6.3). In both cases, the objective is to reduce pressure by increasing the area over which the weight acts.

[Figure 6.2: Buckets with broad and narrow handles, See in your textbook]

[Figure 6.3: Persons carrying loads, See in your textbook]

Pressure is defined as the force per unit area. The SI unit of force is newton and that of area is metre\( ^2 \). Therefore, the SI unit of pressure is newton/metre\( ^2 \) (N/m\( ^2 \)). This unit is also called a pascal, denoted by Pa.

If a force of 100 N is applied on a cardboard of area 2 m\( ^2 \), then the pressure applied on the cardboard will be:

\[ \text{Pressure} = \frac{\text{Force}}{\text{Area}} = \frac{100 \text{ N}}{2 \text{ m}^2} = 50 \text{ N/m}^2 \]

There are many situations in daily life where pressure plays a role. Conduct the activities given in Table 6.1 and record your observations. Explain how pressure influences the mode of action undertaken for each activity.

Safety first

The activities listed in Table 6.1 should be conducted under the supervision of an adult.

Teacher's Note

Think about why a sharp knife cuts better than a blunt one even with the same force. The sharp edge has a much smaller area in contact with the apple, so it concentrates the force into higher pressure on a tiny spot. A blunt knife spreads the same force over a larger area, creating less pressure, so it cannot cut. This is why nails are pointed - the sharp tip makes pressure large enough to pierce materials.

Key Points

• Pressure is the force acting on a unit area of a surface, calculated as \( \text{Pressure} = \frac{\text{Force}}{\text{Area}} \).
• The SI unit of pressure is the pascal (Pa), which equals newton per square metre (N/m\( ^2 \)).
• For the same force, a smaller area produces higher pressure, while a larger area produces lower pressure. This is why broad bag straps are more comfortable than narrow ones.
• In daily life, we use pressure principles when driving nails with sharp points, cutting with sharp knives, or placing cloth under heavy loads on our heads.

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