Lesson summary
4.3.2 Pressure — AS PHYSICS 9702 notes: Define pressure, calculate it for solids and understand pressure in fluids and hydraulic systems.Pressure = the force acting normally per unit area: p = F ÷ A.Unit — pascal (Pa), with 1 Pa = 1 N m-2.Pressure is a scalar — it has no specific…
- Only the component of force perpendicular to the surface counts
- Use the cross-sectional area in contact with the surface
- A force spread over a large area gives a small pressure
- High-heeled shoes exert greater pressure than flat shoes
- A nail has a pointed tip and a wide head
- Skis, snowshoes and wide tractor tyres
Section 1 of 9Definition
Definition
Pressure
Pressure the force acting normally per unit area: p = F ÷ A.
Unit
- Unit pascal (Pa), with 1 Pa = 1 N m-2.
Pressure is a scalar
- Pressure is a scalar it has no specific direction, unlike force.
Only the component of force perpendicular to the surface counts
- Only the component of force perpendicular to the surface counts a force at angle θ to the surface gives pressure F sin θ ÷ A.
Use the cross-sectional area in contact with the surface
- Use the cross-sectional area in contact with the surface in m2, with 1 cm2 = 10-4 m2.
Atmospheric pressure at sea level is about 1.0 × 105 Pa.
Section 2 of 9Pressure in solids
1A force spread over a large area gives a small pressure
- A force spread over a large area gives a small pressure over a small area it gives a large pressure.
2High-heeled shoes exert greater pressure than flat shoes
- High-heeled shoes exert greater pressure than flat shoes the same weight acts on a smaller area.
3A nail has a pointed tip and a wide head
- A nail has a pointed tip and a wide head and knives are sharpened to raise the pressure.
4Skis, snowshoes and wide tractor tyres
- Skis, snowshoes and wide tractor tyres spread the weight over a large area to reduce the pressure.
Section 3 of 9Pressure in fluids
Pressure in fluids
A fluid (liquid or gas) exerts a force on any surface in contact with it
- A fluid (liquid or gas) exerts a force on any surface in contact with it the force is perpendicular to the surface.
The pressure at a given depth acts equally in all directions.
Pressure in a liquid increases with depth
- Pressure in a liquid increases with depth dams are thicker at the bottom.
Pressure in a gas arises from molecules colliding with surfaces
- Pressure in a gas arises from molecules colliding with surfaces each collision exerts a tiny force.
Pressure is the same at the same horizontal level in a static liquid.
Section 4 of 9Hydraulic systems
Hydraulic systems
A liquid is virtually incompressible.
A pressure applied by a small piston of area A1 with force F1
- A pressure applied by a small piston of area A1 with force F1 acts on a large piston of area A2, giving force F2 = pA2.
F1 ÷ A1
F2 ÷ A2
- so a small force can lift a large load.
The small piston moves further, by the ratio of the areas
- The small piston moves further, by the ratio of the areas the volume pushed in equals the volume pushed out, so no energy is gained.
Car brakes and hydraulic jacks work this way.
Section 5 of 9Force from pressure
1When the pressure is known
- When the pressure is known the force on a surface is F = pA.
2Atmospheric pressure acts on both sides of a table top
- Atmospheric pressure acts on both sides of a table top so the net force is zero and the table is not crushed.
3Differences in pressure produce net forces
- Differences in pressure produce net forces such as on aircraft windows when the cabin pressure is higher than outside.
4Sucking on a straw
- Sucking on a straw reduces the pressure inside, and the outside atmosphere pushes the drink up.
Section 6 of 9Links
Pressure combines with density to give the change of pressure with depth Δp = ρgΔh (see 4.3.3).
It underlies upthrust (see 4.3.4).
In kinetic theory
- In kinetic theory gas pressure arises from molecular collisions.
Section 7 of 9More from the specification
More from the specification
Hydrostatic pressure formula
- The hydrostatic pressure exerted by a fluid column of height h and density rho is given by p = rho g h, acting perpendicular to submerged surfaces.
Archimedes' principle and upthrust
- Any object wholly or partially immersed in a fluid experiences an upward buoyant force equal to the weight of fluid displaced by the object.
Principle of floatation
- A floating cylinder or body displaces a volume of liquid whose total weight exactly equals the downward gravitational weight of the body.
Hydrostatic pressure formula
- The hydrostatic pressure exerted by a fluid column of height h and density rho is given by p = rho g h, acting perpendicular to submerged surfaces.
Archimedes' principle and upthrust
- Any object wholly or partially immersed in a fluid experiences an upward buoyant force equal to the weight of fluid displaced by the object.
Principle of floatation
- A floating cylinder or body displaces a volume of liquid whose total weight exactly equals the downward gravitational weight of the body.
Newton
- Unit of force, kg m s-2.
Manometer
- Device measuring pressure differences with a liquid column.
Hydrostatic pressure
- P = P0 + ρgh.
Gravitational field
- Region where mass experiences gravitational force.
Field strength
- G = Fg/m.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Pressure | The normal force per unit area, p = F ÷ A. |
| Pascal | The SI unit of pressure, 1 N m-2. |
| Atmospheric pressure | The pressure of the air, about 1.0 × 105 Pa at sea level. |
| Hydraulic system | A system that uses a liquid to transmit pressure and multiply force. |
| Incompressible | Unable to be squeezed into a smaller volume. |
| Normal force | A force acting at right angles to a surface. |
| Scalar | A quantity with magnitude only. |
| Piston | A moving part in a cylinder that applies or receives pressure. |
Section 9 of 9Exam technique
1
Using the area in cm2 without converting.
2
Thinking pressure is a vector — it is a scalar.
3
Using the total weight when only part of the force is normal to the surface.
4
Believing a larger force always means a larger pressure — area matters.
5
Thinking hydraulics gain energy — the work done by the load is no more than the work done by the effort.