Example 1: pressure of a box
A 500 N box rests on a base of 0.25 m². Find the pressure.
- Pressure = force ÷ area.
- 500 ÷ 0.25 = 2000 Pa.
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IB DemystifiedMYP Sciences
Pressure in solids, liquids and gases
Why does a sharp knife cut, a camel not sink into sand, and a dam wall grow thicker at the bottom? The answer is pressure: how concentrated a force is. Pressure also explains straws, pressure cookers and why your ears pop on a mountain road.
Recommended for MYP 2 · About 3 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding the ideas
- Calculating pressure
- Pressure in the real world
- Worked examples
- Assessment tips
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- calculate pressure using pressure = force ÷ area
- explain how area changes pressure in everyday examples
- explain why liquid pressure increases with depth
- explain gas pressure using particles
- describe atmospheric pressure and how it changes with height
- discuss dams and pressure-cooker safety
Before you start
You will use these skills. If any feel shaky, review them first.
- forces and weight (see Forces and their effects)
- particles in solids, liquids and gases (see States of matter)
- dividing decimals
Key vocabulary
- Pressure
- How concentrated a force is: force ÷ area.
- Pascal (Pa)
- The unit of pressure: 1 Pa = 1 N/m².
- Area
- The size of a surface, in m².
- Liquid pressure
- Pressure caused by the weight of liquid above; increases with depth.
- Gas pressure
- Pressure caused by gas particles colliding with surfaces.
- Atmospheric pressure
- The pressure of the air around us, caused by the weight of air above.
Understanding the ideas
-
What is it?
Pressure tells us how concentrated a force is. The same force on a small area gives a large pressure (a knife edge, a drawing pin point); on a large area it gives a small pressure (snowshoes, camel feet, tractor tyres).
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Why does it happen?
In a liquid, pressure increases with depth, because there is more liquid above pushing down, and it acts in all directions. In a gas, pressure comes from particles colliding with surfaces; more particles, or faster particles, mean higher pressure.
-
How do we know?
We calculate pressure as force ÷ area, in pascals. Scientists measure air pressure with barometers, and engineers use pressure calculations to design dams, submarines and tyres.
-
Why does it matter?
Understanding pressure keeps us safe, from strong dam walls to working safety valves on pressure cookers and gas cylinders.
-
What does it connect to?
Pressure links to forces and particles in physics and chemistry, to breathing and blood pressure in biology, and to weather.
Calculating pressure
- pressure = force ÷ area (Pa = N ÷ m²).
- Bigger area → smaller pressure (for the same force).
- Smaller area → bigger pressure.
- Liquids: pressure increases with depth and acts in all directions.
- Gases: more particles or higher temperature → more collisions → higher pressure.
Pressure in the real world
Tarbela Dam's wall is far thicker at the base than at the top. Pressure cookers are common in kitchens because food cooks faster at the higher boiling temperature. Mountain travellers on the Karakoram Highway notice their ears pop as air pressure falls.
Worked examples
Example 2: finding force
A pressure of 3000 Pa acts on 0.5 m². What is the force?
- Force = pressure × area.
- 3000 × 0.5 = 1500 N.
Assessment tips
Pressure questions often combine calculations with everyday examples. Expect to:
- Calculate pressure, force or area, with units.
- Explain why area affects pressure.
- Explain liquid and gas pressure using depth and particles.
- Plan fair tests and analyse data.
Common mistakes: multiplying instead of dividing; forgetting to convert areas to m²; saying suction pulls liquids; and thinking liquid pressure depends on the width of a container.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: what pressure do you exert?
Partially guided investigation · about 40 minutes · pairs
- Research question
- What pressure do you exert on the floor when standing on two feet, on one foot, and on tiptoe?
- Scientific background
- Your weight in newtons is about your mass in kg × 10. The area of your shoe can be found by counting squares on squared paper.
- Hypothesis
- Write your own prediction, with a scientific justification.
- Variables
- Identify your independent, dependent and control variables, and explain how you will control them.
- Apparatus
- Bathroom scales, squared paper (1 cm squares), pencil, calculator.
- Method
- Weigh yourself and multiply your mass by 10 to find your weight in newtons.
- Draw round your shoe on squared paper and count the squares (count a square if more than half is inside).
- Convert the area from cm² to m² by dividing by 10 000.
- Calculate the pressure for two feet, one foot, and tiptoe (estimate the area).
Safety. Stand on tiptoe near a wall so you can steady yourself.
Then evaluate: which parts of your measurement were least accurate, and how could you improve them?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
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