Definition
Fluid
A substance that can flow, which means any liquid or gas.
Definition
Pressure
The force acting at right angles to a surface on each unit area of that surface.
- A fluid is any substance that can flow, so all liquids and all gases are fluids.
- The particles in a fluid are free to move, and they are always moving around randomly.
- As they move, the particles collide with any surface in contact with the fluid, such as the wall of a container or the skin of a swimmer.
- Each collision exerts a tiny force on the surface, and the enormous number of collisions every second adds up to a steady force spread across the surface.
- The force on each unit of area of the surface is the pressure of the fluid.
Forces at right angles to surfaces
- The pressure in a fluid causes a net force at right angles to any surface it touches.
- The force is at right angles, or normal, to the surface whichever way the surface faces, whether it is horizontal, vertical or sloping.
- The particles strike the surface from many directions, and the sideways parts of their pushes cancel out, leaving only a push straight into the surface.
- At any point in a fluid, the pressure acts equally in all directions, so a tiny surface placed at that point feels the same force per unit area however it is turned.
- A plastic bottle full of water with small holes punched around its side squirts water out of every hole at right angles to the wall at that hole.
- The air inside a balloon pushes outwards at right angles to every part of the rubber, which is why a balloon inflates into a smooth, rounded shape.
- The water around a submarine pushes inwards at right angles to every part of its hull, so the hull must be strong on its sides and underneath as well as on top.
The net force from fluid pressure
Definition
Resultant force
The single force that has the same effect on an object as all of the forces acting on it combined.
- A surface often has fluid on both sides, and the fluid on each side pushes on it at right angles.
- If the pressure is the same on both sides, the two forces are equal and opposite, so there is no net force on the surface.
- If the pressure is different on the two sides, there is a net force at right angles to the surface, directed from the higher-pressure side towards the lower-pressure side.
- The air in a pumped-up car tyre is at a higher pressure than the air outside, so there is a net outward force on the tyre wall that keeps the tyre firm.
- An aircraft cabin is kept at a higher pressure than the thin air outside at cruising height, so there is a net outward force on every window and door.
- Sucking air out of a plastic drinks bottle lowers the pressure inside it, so the net inward force from the air outside crushes the bottle.
Example
Forces on a balloon
- A balloon is blown up so that the air inside it is at a higher pressure than the air outside.
- The air inside pushes outwards at right angles to the rubber at every point.
- The air outside pushes inwards at right angles to the rubber at every point, but with a smaller force on each unit of area.
- The net force on each part of the rubber is therefore outwards and at right angles to the surface, so the rubber is stretched evenly into a rounded shape.
Example
Water jets from a squeezed bottle
- A plastic bottle filled with water has small holes in its side, in its sloping shoulder and in its base.
- When the bottle is squeezed, water leaves each hole moving at right angles to the surface around that hole.
- The jets leave sideways from the side wall, at a slant from the sloping shoulder and straight downwards from the base.
- The jets show that the force from water pressure acts at right angles to every surface, whichever way the surface faces.
Common Mistake
- Do not say that fluid pressure only acts downwards, because it pushes at right angles on surfaces facing in every direction.
- Do not describe suction as a pulling force, because the effect comes from a larger push by the fluid on the higher-pressure side.
- Do not forget that liquids and gases are both fluids, so the same ideas apply to air as to water.
Exam technique
Explaining forces from fluid pressure
- State that the force caused by fluid pressure acts at right angles to the surface, and use the word normal if you prefer.
- Explain pressure using particles, saying that they collide with the surface, each collision exerts a force, and these forces add up over the area.
- When a net force is asked for, compare the pressures on the two sides and say that the net force acts from the higher-pressure side towards the lower-pressure side.
- Use the word fluid to cover both liquids and gases.
Self review
- State what is meant by a fluid.
- Describe the direction of the force that fluid pressure exerts on a surface.
- Explain, in terms of particles, how a gas exerts a force on the wall of its container.
- Explain why water squirts sideways out of a hole in the side of a bottle.
- Describe the direction of the net force on a surface when the pressure on one side is greater than on the other.
