What is Pressure? | Physics for Grade 10 PDF Download

Pressure in a Fluid

• A fluid is either a liquid or a gas
• Pressure is defined as
  The concentration of a force or the force per unit area
• For example, when a drawing pin is pushed downwards:
  • It is pushed into the surface, rather than up towards the finger
  • This is because the sharp point is more concentrated (a small area) creating a larger pressure
    
    When you push a drawing pin, it goes into the surface (rather than your finger)
• Example 1: Tractors
  • Tractors have large tyres
  • This spreads the weight (force) of the tractor over a large area
  • This reduces the pressure which prevents the heavy tractor from sinking into the mud
• Example 2: Nails
  • Nails have sharp pointed ends with a very small area
  • This concentrates the force, creating a large pressure over a small area
  • This allows the nail to be hammered into a wall
• When an object is immersed in a liquid, the liquid will exert pressure, squeezing the object
  • The pressure exerted on objects in fluids creates forces against surfaces
  • These forces act at 90 degrees (at right angles) to the surface
    
    The pressure of a fluid on an object creates a force normal (at right angles) to the surface

Calculating Pressure

• The pressure at the surface of a fluid can be calculated using the equation:
  
• Pressure is measured in the units Pascals (Pa)
• The area should always be the cross-sectional area of the object
  • This means the area where the force is at right angles to it
• This equation can be rearranged with the help of a formula triangle:
  
• This equation means:
  • 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
    
    High heels produce a higher pressure on the ground because of their smaller area, compared to flat shoes

Exam Tip

Solved Example

The pump creates pressure in the liquid of 5.28 × 10⁵ Pa to move the platform upwards. Calculate the force that the liquid applies to the piston.

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 = F/A
Step 3: Rearrange for the force, F
F = p × A
Step 4: Substitute the values into the equation
F = (5.28 × 105) × (2.73 × 10-2) = 14 414.4
Step 5: Round to the appropriate number of significant figures and quote the correct unit
F = 14 400 N = 14.4 kN (3 s.f)