Pressure & Forces (Cambridge International IGCSE Physics)

Pressure

Spec point — Define pressure as force per unit area; recall and use the equation p = F/A Describe how pressure varies with force and area in the context of everyday examples

Pressure

What is pressure?

• Pressure is defined as

The force per unit area

• Pressure is measured in newtons per metre squared (N/m²)

  • 1 newton per metre squared is equivalent to 1 pascal (Pa)
  • 1 N/m² = 1 Pa

• It can be calculated using the following pressure equation:

$\mathrm{pressure}=\frac{\mathrm{force}}{\mathrm{area}}$

$p=\frac{F}{A}$

• Where:

  • p = pressure, measured in newtons per metre squared (N/m²) or pascals (Pa)
  • F = force, measured in newtons (N)
  • A = area, measured in metres squared (m²)

• This equation tells us that:

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

• If an object rests on more than one point of contact, add their areas together to get the total area the force is spread over

Exam Hint In this course, pressure may also be given in N/cm². If a question provides an area in cm², the pressure can be given in N/cm².

Examples of applications of pressure

• Example 1: Tractors

  • Tractors have large tyres
  • This spreads the weight (force) of the tractor over a large area
  • The weight of the tractor is unchanged, so this reduces the pressure and prevents the heavy tractor from sinking into the mud

• Example 2: Drawing pins

  • Drawing pins have sharp pointed ends with a very small area
  • This concentrates the force, creating a large pressure over a small area
  • This allows the drawing pin to be pushed into a wall

Applying a force to a drawing pin

When you push a drawing pin, it goes into the surface (rather than your finger)

• Example 3: High heels

  • High heels have small, sharp points with a small area
  • This concentrates the weight (force), creating a large pressure over a small area
  • Flat shoes have a larger area which the weight (force) is spread over resulting in a lower pressure
  • If the person's weight is the same in both shoes, this explains why high heels sink into soft surfaces more easily than flat shoes

The effect of surface area on pressure

High heels produce a higher pressure on the ground because of their smaller area, compared to flat shoes

Exam Hint Note that answers about friction, balance or stability score nothing on a pressure question. Make sure to name the area, the equation and the resulting pressure.

Worked Example The diagram below shows the parts of the lifting machine used to move the platform up and down.

The pump creates pressure in the liquid of 5.28 × 10⁵ Pa to move the platform upwards. The cross-sectional area of the piston is 2.73 × 10^-2 m².

Calculate the force that the liquid applies to the piston.

[2]

Answer:

Step 1: List the known quantities

• Cross-sectional area = 2.73 × 10^-2 m²
• Pressure = 5.28 × 10⁵ Pa

Step 2: Write down the relevant equation

$p=\frac{F}{A}$

Step 3: Rearrange for the force, F

$F=p\times A$

Step 4: Substitute the values into the equation

$F=(5.28\times10^{5})\times(2.73\times10^{-2})=14414.4$ [1 mark]

Step 5: Round to the appropriate number of significant figures and quote the correct unit

$F=14400N=14.4\mathrm{kN}(3s.f.)$ [1 mark]

Worked Example A table stands on four legs, and each leg has an area of 100 cm². The table has a weight of 120 N.

Calculate the pressure on the ground.

[4]

Answer:

Step 1: List the known quantities

• Area of one table leg = 100 cm²
• Weight = 120 N

Step 2: Write down the relevant equation

$p=\frac{F}{A}$ [1 mark]

Step 3: Calculate the total area of all four table legs

$A=4\times100=400\mathrm{cm}^{2}$ [1 mark]

Step 4: Substitute the values into the equation

$p=\frac{120}{400}$ [1 mark]

$p=0.3N/\mathrm{cm}^{2}$ [1 mark]

Pressure Force & Area video

Spec point — nan