Penerangan
Defining Pressure in Solids
Pressure is a measure of how concentrated a force is when it is applied to a surface. When a force pushes against a surface, that force is spread out over a specific contact area. The magnitude of the pressure depends entirely on two factors: the size of the applied force and the area over which that force acts. If the same force is applied over a smaller area, the pressure increases significantly because the force is concentrated into a narrower space. Conversely, if that same force is spread out over a larger area, the pressure decreases because the force is distributed more widely.
The scientific definition of pressure is the perpendicular force exerted per unit area. This relationship is expressed using the standard equation: pressure = force / area. In the International System of Units, force is measured in newtons (N) and area is measured in square metres (m2). Consequently, the standard unit of pressure is newtons per square metre, which is given the special name of the pascal (Pa). One pascal is equal to a pressure of one newton acting over an area of one square metre. In many practical situations involving larger pressures, kilopascals (kPa) are used, where one kilopascal equals one thousand pascals.
Understanding this mathematical relationship explains many everyday phenomena involving solid objects. For instance, sharp knives slice easily through food because the blade has an extremely small surface area at its cutting edge. Even when a person applies a modest force with their hand, the tiny area of the blade concentrates that force into a very high pressure, allowing the blade to sever material easily. In contrast, heavy transport vehicles, tractors, and military tanks are often fitted with very wide tyres or continuous metal tracks. These large contact areas distribute the massive weight of the vehicle over a much greater surface area, reducing the pressure on the ground and preventing the vehicle from sinking deeply into soft mud or sand.
Calculating Pressure and Working with Units
Solving quantitative problems involving pressure requires careful attention to the units of measurement used for force and area. The formula pressure = force / area relies on force being measured in newtons and area in square metres. If an area is given in square centimetres, it must be converted into square metres before performing the calculation, because one square metre equals ten thousand square centimetres. Failing to convert units correctly is a frequent source of error in numerical problems.
Consider an object resting on a flat surface. The force pushing down on the surface is typically the weight of the object, which is calculated as mass multiplied by gravitational field strength, though force values are often provided directly in newtons in introductory problems. When calculating the pressure exerted by a standing block, identifying the correct contact area is crucial. A rectangular block has three different pairs of faces with different surface areas, meaning the block will exert three different pressures depending on which face is resting on the ground.
Engineers and architects apply these calculations constantly to ensure structures are safe. Foundations of buildings are made very wide so that the immense weight of the entire building creates a low enough pressure on the underlying soil to prevent subsidence. Similarly, snowshoes have a large surface area that prevents a person from sinking into deep snow by distributing their body weight over a wider region.
Pressure in Liquids
Liquids are states of matter where particles are closely packed together but remain free to move past one another. Because liquid particles are in constant, random motion and collide with each other and with the walls of their container, they exert pressure. Unlike solids, which exert downward forces due to gravity acting on a fixed shape, liquids can flow, meaning liquid pressure acts in all directions: downwards, upwards, and sideways against any surface touching the liquid.
Particle theory explains that liquid pressure increases with depth. Deeper inside a liquid, there is a greater weight of liquid pressing down from above due to the pull of gravity. This added weight packs the lower layers of particles slightly closer together and increases the frequency and force of particle collisions against container walls. A simple demonstration of this involves making holes at different heights in the side of a container filled with water. Water squirts out furthest from the hole closest to the base, proving that the pressure at the bottom of the column is higher than near the top.
Another key characteristic of liquid pressure is that at any given depth, the pressure acts equally in all directions. Divers experiencing deep water feel this pressure pressing in uniformly across their entire body. Furthermore, liquids are virtually incompressible because their particles are already touching, which means that an increase in pressure applied at one point in an enclosed liquid is transmitted throughout the entire liquid without loss, a principle utilised in hydraulic braking systems.
Pressure in Gases and Atmospheric Pressure
Gases consist of particles that possess a great deal of kinetic energy and are widely separated by large empty spaces. These particles move rapidly in completely random directions at high speeds, colliding frequently with one another and with the internal surfaces of any container they occupy. Each individual collision exerts a tiny force on the surface. Atmospheric pressure and gas pressure inside closed containers are both the result of the combined effect of billions of these microscopic collisions occurring every second.
When a gas is heated inside a sealed container, the thermal energy increases the average speed of the gas particles. Because the particles are moving faster, they collide with the container walls more frequently and with greater force, causing the gas pressure to rise. If the container is flexible, such as a balloon, the increased internal pressure causes the container to expand until the internal pressure balances the external pressure. Conversely, compressing a gas by reducing the volume of its container forces the particles into a smaller space, increasing the frequency of collisions and raising the pressure.
Earth is surrounded by a thick layer of air known as the atmosphere, extending many kilometres upwards. Gravity pulls these gas particles downward toward the surface of the planet. Atmospheric pressure is the pressure exerted by the weight of this column of air pressing down on Earth. At sea level, atmospheric pressure is substantial, though human bodies do not feel crushed because fluids inside the body exert an equal and opposing outward pressure. As altitude increases and a person climbs a high mountain, there is less air above them, meaning the air is thinner, there are fewer gas particles, and atmospheric pressure decreases noticeably.
