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Pressure (Cambridge (CIE) AS Physics): Revision Note

Leander Oates

Last updated

Pressure

  • Pressure tells us how concentrated a force is

  • Pressure is defined as:

    The force per unit area

  • Pressure is calculated using the following equation:

p space equals space F over A

  • Where:

    • p = pressure in pascals (Pa)

      • 1 Pa = 1 N m-2

    • F = force in newtons (N)

    • A = area in metres squared (m2)

  • Pressure, unlike force, is a scalar

  •  Therefore pressure does not have a specific direction

  • If a force is spread over a large area it will result in a small pressure

  • If a force is spread over a small area it will result in a large pressure

Pressure of different styles of shoes

Pressure on different areas, downloadable AS & A Level Physics revision notes

High-heeled shoes exert a greater pressure than flat shoes because the same force (weight) is spread over a smaller area

Worked Example

A cylinder is placed on a horizontal surface as shown below.

The mass of the cylinder is 4.7 kg and the diameter is 8.4 cm.

Calculate the pressure produced by the cylinder on the horizontal surface.

Answer:

Step 1: List the known quantities and convert to SI units

  • Mass, m = 4.7 kg

  • Diameter of cylinder, d = 0.084 m

Step 2: State the pressure equation

p space equals fraction numerator space F over denominator A end fraction

Step 3: Calculate the force exerted by the cylinder

  • The force is the weight acting on the cylinder

  • Acceleration of free fall, g = 9.81 m s-2

F space equals space W space equals space m g

W space equals space 4.7 space cross times space 9.81

W space equals space 46.1 space straight N

Step 4: Calculate the area 

  • Only the base of the cylinder is in contact with the surface

area space of space circle space equals space straight pi r squared

A space equals space straight pi open parentheses d over 2 close parentheses squared

A space equals straight pi open parentheses fraction numerator 0.084 over denominator 2 end fraction close parentheses squared

A space equals space 5.5 cross times 10 to the power of negative 3 end exponent space straight m squared

Step 5: Substitute the known values into the pressure equation to calculate

p space equals space fraction numerator 46.1 over denominator 5.5 cross times 10 to the power of negative 3 end exponent end fraction

p space equals space 8400 space Pa space open parentheses 2 space straight s. straight f. close parentheses

Examiner Tips and Tricks

The area referred to is the ‘cross-sectional’ area of a 3D object. This is the area of the base that the force is applied on. For a cylinder, this will be a circle.

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    Leander Oates

    Author: Leander Oates

    Expertise: Physics

    Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.