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1.9.1Motion, Forces and Energy

Pressure and Forces — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 90 of 91 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

1.9.1 Pressure and Forces — IGCSE PHYSICS notes: Define pressure and use p = F ÷ A to explain and calculate the effect of force and area in everyday situations.Pressure = the force per unit area, a measure of how concentrated a force is.Equation — p = F ÷ A.What the symbols mean:…

  • Other units
  • Area conversion
  • Several contact points
  • The force is often the weight of an object, W
  • Rearranged forms
  • Convert to base units first

Part 1 of 7What pressure is

Pressure
Pressure the force per unit area, a measure of how concentrated a force is.
Equation
  • Equation p = F ÷ A.
    What the symbols mean: p = pressure (Pa = N m−2) · F = force acting at right angles to the surface (N) · A = area (m2)
  • p is pressure in pascals (Pa), where 1 Pa = 1 N/m².
  • F is force in newtons (N), acting at right angles (normal) to the surface.
  • A is area in square metres (m²).
Other units
  • 1 kPa = 1000 Pa and N/cm² is used when the area is in cm².
Area conversion
  • 1 m² = 10 000 cm².
Several contact points
  • Several contact points add their areas to get the total area.

Part 2 of 7Changing the pressure

1
For a fixed force, a smaller area gives a larger pressure.
2
For a fixed area, a larger force gives a larger pressure.
3The force is often the weight of an object, W
The force is often the weight of an object, W mg.
What the symbols mean: W = weight (N) · m = mass (kg) · g = gravitational field strength (9.81 N kg−1 on Earth)
4
A rectangular block exerts a smaller pressure standing on its largest face than on its smallest face, with the same weight.

Part 3 of 7Large pressure — small area

Large pressure — small area
Knives, nails, drawing pins and needles have sharp points or edges so a modest force gives a very high pressure.
High heels concentrate the weight on a small area, so they sink into soft surfaces more easily than flat shoes.

Part 4 of 7Small pressure — large area

1
Skis, snowshoes, tractor tyres, caterpillar tracks and wide bag straps spread the force over a large area.
2
A tractor with large tyres is prevented from sinking in mud.
3
A camel's wide feet stop it sinking in sand.
4
Tall buildings need wide foundations to keep the pressure on the ground low.

Part 5 of 7Rearranging and using the equation

1Rearranged forms
  • F = pA and A = F ÷ p.
2
Pressure of 200 kPa on an area of 0.020 m² gives a force of 4000 N.
3Convert to base units first
  • Convert to base units first divide an area in cm² by 10 000 to get m².
4
Atmospheric pressure of about 100 000 Pa gives about 10 000 N on each square metre, yet we do not feel it because the pressure inside our bodies pushes outwards equally.

Part 6 of 7Pressure and the particle model

1
A fluid exerts pressure through many tiny collisions of moving particles with the surface.
2
Each collision gives a tiny push, and countless collisions per second give a steady force.
3
Fluid pressure acts in all directions and at right angles to the surface.
4
Car tyres are inflated above atmospheric pressure so the air inside pushes outwards on the tyre wall.
5
A bed of nails spreads the weight over hundreds of points, so the pressure on each is small.

Part 7 of 7More from the specification

More from the specification
Pressure in a Liquid Column
  • The hydrostatic pressure exerted at depth within a liquid depends on the density of the liquid and the vertical height of the column, calculated using Delta p = rho * g * Delta h.
Influence of Depth and Liquid Density
  • Pressure in fluids increases uniformly with depth due to the weight of the liquid column above;
    • substituting relevant quantities shows that dense liquids exert substantially higher pressure than water at identical depth.
Definition of Pressure
  • Pressure is defined as the normal force applied per unit contact area (p = F / A), measured in pascals (Pa) or newtons per square meter (N/m2.
Hydraulic Pressure Transmission
  • Any change in pressure applied to an enclosed incompressible liquid is transmitted equally and undiminished in all directions throughout the liquid system.
Liquid
  • State with a fixed volume that takes the shape of its container.
Density
  • Mass per unit volume.
Liquid pressure
  • Pressure at a point caused by the weight of liquid above it.

Key terms

TermMeaning
PressureForce acting normally per unit area; p = F ÷ A.
Pascal (Pa)Unit of pressure; 1 Pa = 1 N/m².
Normal forceForce at right angles to a surface.
AreaSize of the surface in contact, in m².
WeightForce of gravity on a mass; W = mg.
Atmospheric pressurePressure caused by the weight of air above, about 100 kPa at sea level.

Avoid these mistakes

1
Writing pressure in N instead of Pa — N is the unit of force.
2
Converting cm² to m² incorrectly — divide by 10 000.
3
Believing a larger area gives a larger pressure — it gives a smaller one.
4
Using mass in kg instead of weight in N.
5
Ignoring that the area must be the area in contact.

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