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Physics: 5. Pressure
Syllabus
OP10 State the relationship between pressure, force and area; perform simple calculations using this
relationship
OP11 Investigate the relationship between pressure and depth for a liquid
OP12 Show that air has mass and occupies space
OP13 Associate change in the pressure exerted by the atmosphere with change in altitude
OP14 Examine weather charts to observe variations in atmospheric pressure and relate these to weather
conditions
Student Notes
Pressure is caused by a force that pushes against an object.
The pressure increases when the force increases or when the area (that the force is acting on) decreases.
The unit of pressure is the pascal (Pa).
You can also use the N/m2 or N/cm2.
Example 1:
The wind exerts a horizontal force of 1000 N on a wall of area 20 m2. Calculate the pressure at the wall.
Answer: P = F ÷ A = 1000 ÷ 20 = 50 pascals.
Example 2:
A lady weighs 800 N and is standing in a pair of stiletto heels, each of area 2 cm2.
An elephant weighs 27000 N and the total area of its feet is 1800cm2.
Which exerts more pressure on the ground – the lady in stilettos or the elephant?
Answer
Total pressure exerted by the lady’s (two) feet = 800 ÷ 4 = 200 N/cm2
Total pressure exerted by the elephant’s (four) feet = 36000/1800 = 20 N/cm2
The lady exerts ten times more pressure on the ground than the elephant.
Demonstration
Set up as shown.
The water coming out of the bottom hole travels the farthest, because it is
under the greatest pressure.
Pressure in Liquids: the pressure in a fluid increases with depth.
