9.5. Thrust and Pressure

Have you ever wondered why a camel can run in a desert easily? Why an army tank weighing more than a thousand tonne rests upon a continuous chain? Why a truck or a motorbus has much wider tyres? Why cutting tools have sharp edges? In order to address these questions and understand the phenomena involved, it helps to introduce the concepts.

Thrust is the force acting perpendicular to the surface of an area. It can be visualized by imagining a camel walking on soft desert sand. The camel's feet exert thrust on the ground, and because its weight is distributed over a larger area than just your feet, this helps it move without sinking.

Consider how a person stands on the ground. If they stand upright, they only exert thrust through their feet, which might sink in soft sand. However, if they lie down, their weight is distributed over a larger area and does not sink as much. This principle applies to various situations with different objects.

When you stand on loose sand, the force, that is, the weight of your body is acting on an area equal to the area of your feet. When you lie down, the same force acts on an area equal to the contact area of your whole body, which is larger than the area of your feet. Thus, the effects of forces of the same magnitude on different areas are different.

Pressure is defined as thrust per unit area. When an object with a certain weight is on a larger surface area, the pressure exerted on the ground is lower compared to when the same weight is concentrated on a smaller area. This is why a nail, which has a pointed tip, can penetrate surfaces effectively—it exerts a larger pressure due to its small contact area.

Imagine pressing a sharp knife against a piece of fruit. The pressure is concentrated at the very edge of the knife, allowing it to cut through easily. Conversely, if you tried to press a blunt object, like the handle of a spoon, into the fruit, the larger area means less pressure and it wouldn’t penetrate.

All liquids and gases are fluids. A solid exerts pressure on a surface due to its weight. Similarly, fluids have weight, and they also exert pressure on the base and walls of the container in which they are enclosed. Pressure exerted in any confined mass of fluid is transmitted undiminished in all directions.

Fluids, like liquids and gases, exert pressure in all directions because their molecules move freely. When you apply pressure to a fluid in a closed container, that pressure is distributed evenly throughout the fluid. For example, when you squeeze a balloon, the pressure increases from the point of your squeeze outward through the entire balloon.

Think about squeezing a tube of toothpaste. No matter where you apply the force on the tube, toothpaste comes out from the opening at the other end. This shows how pressure can be transmitted through fluids.

The upward force exerted by water on a bottle is known as upthrust or buoyant force. In fact, all objects experience a force of buoyancy when they are immersed in a fluid. The magnitude of this buoyant force depends on the density of the fluid.

Buoyancy is the ability of an object to float in a fluid. This upward force, or upthrust, acts against the downward force of gravity. If an object is denser than the fluid, it will sink. If it is less dense, it will float. For example, boats float because they displace a volume of water equivalent to their weight, while a rock sinks because it displaces less water than its weight.

Think of a beach ball versus a bowling ball. The beach ball is large, but its material is light and has a lower density than water, so it floats. The bowling ball, however, has a high density and displaces much less water for its weight, causing it to sink.

Objects of density less than that of the liquid in which they are immersed float on the liquid. If the density of the object is more than the density of the liquid in which it is immersed, then it sinks in the liquid.

The concept of buoyancy explains why some objects float while others sink. When an object is placed in a fluid, if the fluid's upthrust (or buoyant force) is greater than the object's weight, it floats. Conversely, if the weight is greater, the object sinks. This relationship can easily be demonstrated with everyday objects such as corks and nails.

Imagine trying to float a marble in water versus a piece of cork. The marble sinks because it is denser than the water, while the cork, being less dense, floats. This principle helps us understand various phenomena, like why ships, despite being made of heavy metal, can float on water.

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