KS3 · Physics
Pressure (force over area)
Press your thumb on a wooden board: nothing happens. Put a drawing pin under it and push just as hard, and the pin slides straight in. Same push. What changed?
What makes a pressure big?
The curve shows the pressure one force makes for every area it could act on. Before you touch anything, predict: if you pile on more force, will the pressure on 0.25 m² go up or down? Now lift the point to add force, 600 N at a time, and watch the whole curve rise. Then read along the curve: the same force on a smaller area (left) gives a far higher pressure; spread over a bigger area (right), the pressure falls.
Predict, then check
Ask two separate questions: how big is the force at each end, and how much area does it act on?
You hammer a nail into a plank of wood. Compare the nail's flat head with its sharp point. Which is true?
Concentrate it or spread it?
Does each one squeeze the force onto a small area, or spread it over a large area? Place each item, then read why.
Still to sort
Small area → high pressure (0)
The force is concentrated on a tiny area.
Where the line is: What decides the group is the area the force acts on, not how big the force is.
Large area → low pressure (0)
The force is spread over a large area.
Where the line is: A heavy person can still make a low pressure, if their weight is spread over a big enough area.
Every one of these is using pressure on purpose. Decide which way each one works.
Work it out
A crate presses down on the floor with a force of 2,400 N, and the area in contact with the floor is 0.25 m². Pick out the force and the area, and see the pressure. Then a 600 N weight is put on top of the crate: forces add, so change the force to 2,400 + 600 = 3,000 N and watch what the pressure does.
Finding pressure (Pa)
p = F ÷ A
Answer
p = 2400 ÷ 0.25
= 9600 Pa
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
The same push can barely mark a surface or go straight through it. The difference is the area it acts on.
What you need to know
- Pressure is how concentrated a force is: the force acting on each unit of area, pushing at right angles to the surface.
- pressure = force ÷ area, or p = F/A. Force is in newtons (N); area is in square metres (m²) or square centimetres (cm²).
- A bigger force on the same area gives a bigger pressure. The same force on a smaller area gives a bigger pressure too.
- Pressure is measured in N/m² or N/cm². One newton per square metre is called a pascal (Pa), and 1 m² = 10,000 cm².
The big picture
Pressure is how concentrated a force is: the force acting on each unit of area, at right angles to the surface. pressure = force ÷ area (p = F/A). More force on the same area gives more pressure; the same force on a smaller area gives more pressure too, and spreading it over a larger area lowers it. Pressure is measured in N/m² (the pascal, Pa) or N/cm², and 1 m² = 10,000 cm².
Key points
Worked example
Problem
A box pushes down on a table with a force of 15 N. The area of the box touching the table is 300 cm². Calculate the pressure the box exerts on the table.
⚠ Watch out
Treating force and pressure as the same thing. A bigger force doesn't automatically mean a bigger pressure: a large force spread over a large area can make less pressure than a small force on a tiny point. Always ask: how much force, on how much area?
Memory hook
Picture the area as a grid of little squares, each taking its share of the force. Pressure is each square's share. A pin point has hardly any squares, so each takes a huge share; a snow shoe has loads, so each gets a tiny one.
Check yourself
A 60 N box rests on a 0.5 m² face. What pressure does it exert? Tip it onto a 0.2 m² face: up or down, and to what? (120 N/m², then 300 N/m².)
Flashcards
(16)What is pressure?
What is the equation for pressure?
Which way does the force act when it makes a pressure on a surface?
What units are force and area measured in, for pressure calculations?
What units is pressure measured in?
What is a pascal (Pa)?
The same force acts on a smaller area. What happens to the pressure?
A bigger force acts on the same area. What happens to the pressure?
Are force and pressure the same thing?
On a hammered nail, where is the force bigger: the head or the point?
Why can a pin or a dart point damage a surface so easily?
Why does a rescue team lie flat, or use boards, on thin ice?
An extra weight is put on top of a load. How do you find the new force?
How many square centimetres are in one square metre?
How do you convert a pressure between N/m² and N/cm²?
Measuring the pressure of your hand: how do you get the area and the force?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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