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PHYSICS · Stage 8 · Chapter 6: Forces, Motion and Pressure

Physics 6.14 Pressure — Force, Area and Pascals (P = F / A)

5 min read 5 parts Lesson 14 of 19

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Physics 6.14 Pressure — Force, Area and Pascals (P = F / A) — Alhussan International Academy — Physics (Stages 7–9) notes: Explain what causes pressure and calculate it from force and area.Pressure = a measure of the force exerted by an object or substance over a certain area.For…

  • Earthmover
  • Rearranged
  • Checking units
  • Force on a horizontal surface
  • Increase pressure
  • Decrease pressure

Part 1 of 5What pressure is

  • Pressure = a measure of the force exerted by an object or substance over a certain area.
  • Force and area — The same force produces a large pressure on a small area and a small pressure when spread over a large area.
  • Earthmover — Wide tracks spread its huge weight over a big area, so it does not sink in mud.
    • A person with a much smaller weight can sink because the area of their feet is small.

Part 2 of 5Pressure equation

  • Equation — Pressure = force ÷ area, or P = F ÷ A.
  • Rearranged — Force = pressure × area, and area = force ÷ pressure.
  • Units — Force is in newtons (N), area in square metres (m²) and pressure in N/m² or pascals (Pa).
    • 1 N/m² = 1 pascal (Pa).
    • If area is in cm² and force in N, pressure is in N/cm².
    • 1 N/cm² = 10 000 Pa and 1 m² = 10 000 cm².
  • Checking units — Writing the units next to each number shows which pressure unit is needed.

Part 3 of 5Method for a pressure calculation

  1. Write down the equation P = F ÷ A.
  2. Convert the area to m² if the answer is wanted in pascals.
  3. Substitute the force and area with their units.
  4. Calculate and give the answer with the correct unit.
  • Force on a horizontal surface — The force is the weight, W = m × g, with g about 10 N/kg.

Part 4 of 5Changing pressure

1Increase pressure1 point
— Reduce the area for the same force.
  • The studs on football boots have a small area, so the pressure is larger and they sink in to give grip.
2Decrease pressure1 point
— Increase the area for the same force.
  • The tracks of an earthmover have a large area, so the pressure on the mud is small.
SituationForceAreaPressure
Earthmover on wide tracksVery largeVery largeSmall, so it does not sink
Person on bare feet in mudSmallSmallLarger, so feet sink
Sharp knife edgeModerateTinyVery large, so it cuts

Part 5 of 5Typical pressure values

  • Atmospheric pressureAbout 100 000 Pa (100 kPa), where 1 kPa = 1000 Pa.
  • Person standingA person on two feet exerts about 15 000 Pa, and standing on one foot doubles it.
  • Sense checkA very small or very large answer may mean units such as cm² and m² were not converted.

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