Pressure

(Triple Only)

Pressure is defined as how much force is being applied over a certain area.

Formula:

• Pressure is directly proportional to the force applied ; as the force increases, pressure increases

• Pressure is inversely proportional to the area ; as the area increases, the pressure decreases

• Pressure always pushes perpendicular to whatever surface it's acting on.

Example: If you press your finger onto a table, you're applying a force over the area of your fingertip. But if you press a sharp pencil onto the table with the same force, it makes a much bigger dent because all that force is concentrated on a tiny area. If the area of your fingertip is , and the force applied is , the pressure would be:

Calculating Pressure at the Surface of a Fluid

A fluid is any substance that can flow, this includes both liquids (like water, oil, and juice) and gases (like air, helium, and carbon dioxide). So when we're talking about pressure in fluids, we're not just talking about liquids - we're talking about anything that can flow and doesn't have a fixed shape.

We can use the same equation for calculating pressure in fluids as pressure in solids:

When you have a column of liquid (like water in a swimming pool or the ocean), the pressure increases the deeper you go according to the equation below:

Formula:

Why Does Pressure Increase with Depth?

When you're deeper in a liquid, there's more liquid above you. More liquid means more weight pushing down on you, which means more force acting on the area of your body (or the object). Since , more force means more pressure.

The density matters too. A denser liquid (like seawater compared to fresh water, or honey compared to water) has more mass in the same volume, so it weighs more. More weight above you means more pressure.

Upthrust

Upthrust is a consequence of pressure increasing with depth. When an object is submerged in a fluid (partially or totally), it experiences different pressures on its top and bottom surfaces.

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The bottom surface is deeper in the liquid, so it experiences a greater pressure (remember, pressure increases with depth). The top surface is shallower, so it experiences less pressure. Since pressure causes a force perpendicular to surfaces:

• The bottom surface has a larger upward force acting on it

• The top surface has a smaller downward force acting on it

• The difference between these forces creates a resultant force upwards

• This upward resultant force is called upthrust

Factors That Influence Floating and Sinking

Whether an object floats or sinks depends on two forces:

• The weight of the object (acting downwards)

• The upthrust from the fluid (acting upwards)

• If upthrust weight: The object floats (or rises up through the fluid).

• If upthrust weight: The object stays where it is (neutral buoyancy - like a submarine hovering)

• If upthrust weight: The object sinks

The atmosphere is a thin layer of air surrounding the Earth, about in depth. The atmosphere gets less dense as you go higher up.

Atmospheric pressure is created by air molecules colliding with surfaces. Every time an air molecule bounces off your skin, it exerts a tiny force. When you add up all the billions and billions of collisions happening every second, you get atmospheric pressure.

Why Atmospheric Pressure Decreases with Height

At sea level, there's a lot of air above you - the entire atmosphere is pressing down. But as you climb a mountain or go up in a plane, there's less air above you. Less air above -> less weight of air -> less force acting on surfaces -> lower pressure.

Think of it like being at the bottom of a swimming pool versus being near the surface. At the bottom, you have the whole depth of water above you creating pressure. Near the surface, there's hardly any water above you, so less pressure. The same principle applies to the atmosphere.

As you go higher:

• The number of air molecules above you decreases

• The weight of air above you decreases

• Therefore the atmospheric pressure decreases

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At sea level, atmospheric pressure is about (or ). It's equivalent to having a weight of about pressing on every square centimetre of your body. You don't notice it because the pressure inside your body balances it out.