What is pressure, and how does it depend on the force applied and the area it acts over?
Pressure: the definition of pressure as force per unit area, the relationship linking pressure, force and area, the unit of pressure, and everyday and gas-pressure examples.
An SQA National 5 Physics answer on pressure, covering the definition of pressure as force per unit area, the relationship p equals F over A, the pascal as the unit of pressure, why a small area gives a high pressure, and how this links to gas pressure and everyday examples.
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What this key area is asking
The SQA wants you to define pressure as force per unit area, use the relationship , state the unit of pressure (the pascal), and explain everyday and gas-pressure situations in terms of pressure.
What pressure means
A heavy object resting on a small area produces a high pressure, while the same object on a large area produces a low pressure. So pressure depends on both the size of the force and the area it is spread over.
The pressure relationship
You can rearrange to find the force, , or the area, . Take care with the area: it must be in square metres, so convert from if necessary (). When an object rests on the ground, the force is its weight, .
Small area, large pressure
Pressure in gases
Pressure also describes gases, linking this key area to the gas laws. In a gas, pressure comes from the countless particles hitting the container walls; the total force of these collisions divided by the wall area gives the gas pressure. The gas laws ( and the temperature relationships) all describe how this pressure changes, and they use the same pascal unit, so understanding underpins them.
Pressure in liquids and the atmosphere
Pressure is not only about solids resting on surfaces. A liquid presses on the walls and base of its container, and the deeper you go the greater the pressure, because there is more liquid weighing down from above. This is why a dam is built thicker at the bottom and why your ears hurt at the bottom of a swimming pool. The air around us also exerts a pressure, called atmospheric pressure, of about at sea level, caused by the weight of the air above pressing down. We do not normally notice it because it acts equally in all directions.
Everyday pressure in numbers
A few everyday figures help to make pressure feel real. Standing on one foot might give a pressure of around ; a sharp drawing pin pushed with a small force can reach millions of pascals at its tip, which is why it pierces a board so easily; and a car tyre is typically inflated to about above atmospheric pressure. Comparing these shows how strongly pressure depends on the area, since the forces involved are often similar but the areas differ enormously.
Try this
Q1. State the relationship between pressure, force and area, and the unit of pressure. [2 marks]
- Cue. ; the unit is the pascal (Pa), equal to .
Q2. A force of acts on an area of . Calculate the pressure. [2 marks]
- Cue. .
Q3. Explain why skis stop a skier sinking into soft snow. [2 marks]
- Cue. The large area spreads the weight out, giving a low pressure so the skier does not sink.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style3 marksA box weighs 240 N and stands on the ground on a base of area 0.30 square metres. Calculate the pressure the box exerts on the ground.Show worked answer →
Use the relationship linking pressure, force and area.
Relationship: .
Substitution: .
Markers reward selecting , correct substitution of the weight as the force and the area, and a final answer in pascals ().
SQA N5 style3 marksExplain why a sharp knife cuts more easily than a blunt one, using the idea of pressure.Show worked answer →
A sharp knife has a very small edge area, while a blunt knife has a larger edge area.
For the same downward force, a smaller area gives a much higher pressure, because pressure is force divided by area.
The higher pressure under the sharp edge is more likely to exceed the strength of the material and cut through it. Markers reward linking small area to high pressure for the same force, and concluding that the higher pressure cuts more easily.
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Sources & how we know this
- SQA National 5 Physics Course Specification — SQA (2019)