Pressure

Pressure is the measure of how much force acts on a specific surface area. When a force is applied to an object, the resulting pressure depends directly on the size of the contact area.

The pressure formula and units

Pressure is calculated using the formula P equals F divided by A, where P is pressure, F is force, and A is contact area. When the contact area decreases, the pressure increases, and when the contact area increases, the pressure decreases. The standard SI unit used to measure pressure is the Pascal, which can also be written as Newton per square meter.

Experiments with area and force

• A single sharp pin easily pops a balloon because force applied at one tiny spot creates extremely high pressure.
• Placing a balloon over many pins spreads the force equally across a larger surface area, allowing it to hold added weights without popping.
• Stacking ten coins on one spot makes a cloth sag deeply because high pressure is concentrated there.
• Spreading ten coins across different places on the cloth distributes the force, so the cloth bends only slightly under lower pressure.

Everyday examples of pressure

• Hammering a nail into a wall works because the force is focused on a very tiny point to generate high pressure.
• Cutting a fruit with a knife and chopping wood with an axe rely on small contact areas to create high pressure.
• A doctor giving an injection uses a thin needle to apply force over a small area and pierce the skin.
• Sitting or lying on a bed of nails does not cause pain because the body weight is distributed across a large surface area.

Atmospheric pressure

Air in the atmosphere surrounds the Earth and pushes toward it with force. This force creates what is known as atmospheric pressure. Scientists measure atmospheric pressure using a special device called a barometer. The barometer was invented by an Italian physicist named Evangelista Torricelli.