8.8. Pressure

Is there any relation between pressure and force? Let us find out.

Try to push a nail into a wooden plank by its head. Did you succeed? Try now to push the nail by the pointed end (Fig. 8.12). Could you do it this time?

This chunk introduces the idea of pressure. It asks the reader to consider how easy it is to push a nail into wood depending on which part of the nail is pushed. When pushing the flat head, it may not penetrate the wood, but using the sharp point allows it to drive in. This is because pressure is defined as force applied over a certain area; a small area (the pointed end) creates a greater pressure than a larger area (the flat head) for the same force.

Think of a push pin or a thumbtack. When you press down on the flat head, it doesn't go through the paper. However, if you apply the same force on the point, it easily pierces the paper. This is a direct demonstration of how pressure works!

I now understand why porters place a round piece of cloth on their heads, when they have to carry heavy loads (Fig. 8.13). By doing this they increase the area of contact of the load with their head. So, the pressure on their head is reduced and they find it easier to carry the load.

This chunk explains how increasing the area of contact can reduce the pressure exerted on a surface. By using a cloth to distribute the weight of a heavy load, porters decrease the pressure on their head. The same weight over a larger area results in lower pressure, making it more comfortable to carry.

Imagine carrying a heavy backpack with narrow straps versus wide straps. The wide straps distribute the load more evenly over your shoulders, making it feel lighter, just as the cloth helps the porters. If the straps were thin, the pressure on your shoulders would be greater, making it much harder to carry the backpack.

Note that the area is in the denominator in the above expression. So, the smaller the area, larger the pressure on a surface for the same force. The area of the pointed end of the nail is much smaller than that of its head. The same force, therefore, produces a pressure sufficient to push the pointed end of the nail into the wooden plank.

This piece of information is emphasizing the mathematical relationship between force, area, and pressure. The formula for pressure is pressure = force / area. Hence, if the area decreases while the force remains constant, pressure must increase. This is why sharp tools are more effective compared to blunt ones.

Think about how a knife slices through food. A sharp knife has a very small edge area, which allows for a high pressure when you apply force (like pressing down). On the other hand, if you use a blunt knife, which has a larger edge area, it requires more force to cut through the same material because the pressure is much lower.

Can you explain now why shoulder bags are provided with broad straps and not thin straps? And, why the tools meant for cutting and piercing always have sharp edges?

This chunk is asking the reader to think critically about design choices based on the concept of pressure. Broad straps reduce pressure on the shoulders, making heavy loads easier to carry. Sharp tools have edges designed to concentrate force into small areas, maximizing pressure to effectively perform tasks like cutting or piercing.

Consider how difficult it is to carry a heavy bag with skinny straps compared to a backpack with padded, wide straps. By enhancing comfort through larger surface area, the weight feels less burdensome. Similarly, sharpened kitchen knives cut through food much more easily than dull knives, which require more force and create lower pressure.

Do liquids and gases also exert pressure? Does it also depend on the area on which the force acts? Let us find out.

This question introduces the concept that both liquids and gases apply pressure to their environments. The pressure exerted by a fluid depends on both the height of the fluid above the point in question and the area over which the force is applied, similar to solids. The concept of hydrostatic pressure in liquids, for example, is directly tied to depth.

Consider a swimming pool. The deeper you go, the more pressure you experience. This is because the weight of the water above increases with depth, exerting more pressure on you. Similarly, when you blow up a balloon, the air inside exerts pressure on the walls due to the force of the gas particles moving around.

What do you observe? Does the rubber sheet bulge this time? Pour some more water into the bottle. Is there any change in the bulge of the rubber sheet?

This chunk intrigues the reader to observe the relationship between liquid pressure and the resulting effects on their surroundings. When water is added to the bottle, the rubber sheet distends due to the pressure exerted by the liquid. This gives insight into how pressure works in liquids and how it can be measured through observable effects.

Think of how a balloon reacts when filled with air or water. As you fill it, the surface stretches more. Similarly, when you press down on a water-filled balloon, the water applies pressure in all directions, illustrating that fluids exert pressure consistently in all circumstances.

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