1.9Motion, Forces and Energy
Pressure — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
10 min read 16 parts Lesson 89 of 91 Unit quiz
Lesson summary
1.9 Pressure (Part 1) — IGCSE PHYSICS notes: Define pressure, calculate it from force and area, and explain everyday applications of large and small areas.Definition — pressure is the force per unit area.p = F ÷ A, where p is pressure, F is force and A is area.Units — pressure is…
- Rearrangements
- Drawing pins
- High heels
- Pressure in a liquid pump
- Force on the piston
- Large area gives large force
Section 1 of 16Definition of pressure
Definition
- Definition pressure is the force per unit area.
- p = F ÷ A, where p is pressure, F is force and A is area.
Units
- Units pressure is measured in N/m2, which equals the pascal (Pa).
- 1 N/m2 = 1 Pa, and N/cm2 is also used when area is in cm2.
Rearrangements
- F = pA and A = F ÷ p.
Section 2 of 16What the equation tells us
| Situation | Result |
|---|---|
| Same force, larger area | Smaller pressure |
| Same force, smaller area | Larger pressure |
| Several points of contact | Add their areas to get the total area |
| Double the force, same area | Pressure doubles |
Section 3 of 16Applications
Tractors
- Tractors large tyres spread the weight over a large area, which lowers the pressure so the tractor does not sink into mud.
Drawing pins
- Drawing pins the sharp point has a tiny area, so the force from your thumb makes a large pressure that pushes into the wall.
High heels
- High heels the small heel area gives a high pressure, so they sink into soft ground more easily than flat shoes carrying the same weight.
Section 4 of 16Method for pressure calculations
1
List the force (a weight is mg) and the area.
2
Convert units, such as cm2 to m2 or keep cm2 for N/cm2.
3
Add the areas if there are several contact points.
4
Use p = F ÷ A, or F = pA if the pressure is given.
5
Round to a sensible number of significant figures and state the unit.
Section 5 of 16Hydraulic systems
Hydraulic systems
Pressure in a liquid pump
- Pressure in a liquid pump a pump creates a pressure in the liquid that acts on a piston.
Force on the piston
- Force on the piston the force equals the pressure multiplied by the piston's area.
Large area gives large force
- Large area gives large force this is why hydraulic lifts can raise heavy platforms.
Section 6 of 16More about pressure
1Same weight, different pressure
- Same weight, different pressure a person standing on one foot exerts double the pressure of standing on two.
2Other applications
- Other applications caterpillar tracks, wide tyres, skis and snowshoes all lower pressure with a big area.
3Sharp tools
- Sharp tools knives, nails and needles raise pressure with a tiny area.
4Unit conversions
- 1 m2 = 10 000 cm2, so 1 N/cm2 = 10 000 Pa.
Section 7 of 16More from the specification
1Pressure distribution across fluid boundaries
- Pressure distribution across fluid boundaries static fluids exert isotropic normal forces per unit area perpendicular to container walls.
2Pressure definition and formula
- Pressure is perpendicular force per unit contact area: p = F / A, measured in pascals (1 Pa = 1 N/m2.
3Hydrostatic liquid pressure
- Pressure in liquids increases directly with depth and density: delta p = rho g delta h, acting equally in all spatial directions at a given depth.
4Hydraulic machines and force multiplication
- Liquid incompressibility allows hydraulic systems to transmit pressure equally;
- a small input force on a small piston creates a large output force on a large piston.
5Hydrostatic fluid pressure
- Hydrostatic fluid pressure fluid pressure equation Δp = ρ g h describes how pressure increases linearly with liquid density and column depth.
6Pressure units definition
- Pressure units definition pressure equals force in newtons acting perpendicularly over an area of one square metre, yielding pascals.
7Atmospheric pressure contribution
- Atmospheric pressure contribution total pressure on an object submerged in water equals atmospheric surface pressure added to liquid column pressure.
8Atmospheric pressure
- Pressure of the air, about 1.01 × 105 Pa at sea level.
9Gravitational field
- Region where mass experiences gravitational force.
10Field strength
- G = Fg/m.
11Density
- Mass per unit volume.
12Newton
- Unit of force, kg m s-2.
Section 8 of 16Why liquid pressure depends on depth
Cause
- Cause pressure in a liquid is caused by the weight of the liquid above.
- More depth means more liquid above, so the pressure is greater.
Column of water
- Column of water the highest pressure is at the bottom.
- A hole near the bottom produces a strong jet and a hole near the top a weak jet.
Equal in all directions
- Equal in all directions pressure at the same depth is the same in all directions.
- Holes at the same height spurt water the same distance.
Section 9 of 16Why liquid pressure depends on density
Why liquid pressure depends on density
Denser liquid
- Denser liquid a denser liquid has more mass per volume above a point, so its pressure is greater.
Sea water and fresh water
- Sea water and fresh water sea water is denser, so at the same depth its pressure is higher.
Fluids
- Fluids in all fluids, liquids and gases, pressure is affected by depth and density.
Section 10 of 16Calculating change in pressure
Equation
- Δp = ρgΔh.What the symbols mean: p = pressure due to the liquid (Pa) · ρ = density of the liquid (kg m−3) · g = 9.81 N kg−1 · h = depth (m)
- Δp is the pressure change in pascals.
- ρ is the liquid density in kg/m3.
- g is gravitational field strength in N/kg.
- Δh is the change in depth in metres.
Direction
- Direction liquid pressure acts on every surface of a submerged object at right angles to that surface.
Total pressure
- Total pressure add atmospheric pressure (about 100 kPa) to the liquid pressure.
- Subtract atmospheric pressure when given the total and asked for the liquid pressure alone.
Section 11 of 16Method
1
Decide if the question asks for liquid pressure alone or total pressure.
2
Convert units, such as kPa to Pa.
3
Write Δp = ρgΔh and rearrange for the unknown.
4
Substitute and calculate.
5
Add or subtract atmospheric pressure if needed.
Section 12 of 16Comparing the two pressure equations
| Equation | Use | Depends on |
|---|---|---|
| p = F ÷ A | Pressure from any force on an area | Force and area |
| p = ρgh | Pressure caused by a liquid column | Density, g and depth |
Section 13 of 16Applications and checks
Dam walls
- Dam walls they are thicker at the bottom because liquid pressure is greater at depth.
Submarines
- Submarines deeper dives need stronger hulls because the pressure increases with depth.
Same depth rule
- Same depth rule the pressure at the same depth in the same liquid is equal, whatever the shape of the container.
Density comparison
- Density comparison mercury (13 600 kg/m3) gives about 13.6 times the pressure of water at the same depth.
Units
- 1 kPa = 1000 Pa, and the depth must be in metres.
Section 14 of 16More from the specification
More from the specification
Compressive fluid contact forces
- Compressive fluid contact forces fluid pressure exerts uniform perpendicular contact forces distributed evenly across every square metre of container surfaces.
Pressure definition and fundamental equation
- Pressure is the force exerted perpendicularly per unit surface area (p = F / A),
- where force is measured in newtons (N) and area in metres squared (m²), yielding pascals (Pa = N/m²).
Effect of surface area on exerted pressure
- The same downward weight produces vastly different pressures depending on contact area;
- sharp high heels exert massive pressure,
- whereas flat snow shoes spread force over a large area to reduce sinking.
Calculating pressure with relevant physical quantities
- To calculate pressure accurately, relevant quantities such as weight force (N) and contact area (m²) must be determined from drawing diagrams and dimensions.
Fluid pressure depth dependence
- Pressure exerted by a liquid column increases with depth and density according to the hydrostatic equation Δp = ρgΔh, acting equally in all directions at a given depth.
Pressure definition and fundamental equation
- Pressure is the force exerted perpendicularly per unit surface area (p = F / A),
- where force is measured in newtons (N) and area in metres squared (m²), yielding pascals (Pa = N/m²).
Effect of surface area on exerted pressure
- The same downward weight produces vastly different pressures depending on contact area;
- sharp high heels exert massive pressure,
- whereas flat snow shoes spread force over a large area to reduce sinking.
Calculating pressure with relevant physical quantities
- To calculate pressure accurately, relevant quantities such as weight force (N) and contact area (m²) must be determined from drawing diagrams and dimensions.
Fluid pressure depth dependence
- Pressure exerted by a liquid column increases with depth and density according to the hydrostatic equation Δp = ρgΔh, acting equally in all directions at a given depth.
Newton
- Unit of force, kg m s-2.
Spread
- The extent to which repeated readings differ.
Section 15 of 16Key terms
| Term | Meaning |
|---|---|
| Pressure | Force per unit area. |
| Pascal (Pa) | The unit of pressure, one newton per square metre. |
| Area | The size of a surface, measured in square metres. |
| Hydraulic | Using a liquid under pressure to transmit force. |
| Contact area | The area over which a force is spread. |
| Term | Meaning |
|---|---|
| Liquid pressure | Pressure at a point caused by the weight of liquid above it. |
| Density | Mass per unit volume. |
| Depth | The vertical distance below the surface. |
| Atmospheric pressure | Pressure of the air, about 100 kPa. |
| Fluid | A liquid or a gas. |
Section 16 of 16Exam technique
1
Forgetting to add the areas of all the legs or feet.
2
Using the wrong area unit with N/m².
3
Explaining pressure with friction or stability.
4
Saying a bigger force always means a bigger pressure — the area matters too.
1
Forgetting atmospheric pressure when the total pressure is asked for.
2
Using depth in centimetres in the formula.
3
Thinking pressure depends on the width of the container — it depends on depth and density.
4
Saying liquid pressure only acts downwards — it acts in all directions.
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