Pressure on surfaces, in fluids and in the atmosphere
Why do drawing pins go in and snowshoes stay on top? Why do your ears hurt at the bottom of a pool? One idea, three settings: force spread over an area.
Part of your national curriculum
- Physics: motion and forces: Understand pressure in fluids and atmospheric pressure, and pressure on surfaces
Lesson overview
What you'll learn in this lesson
Understand pressure in fluids and atmospheric pressure, and pressure on surfaces
Key learning points
- • Force spread over an area
- • Doing the calculation
- • Pressure in a liquid
- • Atmospheric pressure
This lesson at a glance
- 30 minutes
- 19 parts to scroll through
- 4 quick checks
- Marked quiz at the end
- Gentle pace: short sittings with pauses
Words to know
Scroll down — the lesson carries on below
Watch & discover
Part 1 of 19
Watch first
Pressure on surfaces, in fluids and in the atmosphere
An English teaching video on this topic from Massimo Boscherini. Watch it, then scroll down for the lesson.
Do it for real
The practicals below use what the film shows, with a full method you can follow at home.
In a nutshell
Understand pressure in fluids and atmospheric pressure, and pressure on surfaces
What you already know
Part 2 of 19
Before we start
What you already know
You should know that force is measured in newtons and that liquids and gases are made of moving particles.
Key words
Part 3 of 19
Key words
Words you'll need today
- Pressure
- Force per unit area, measured in pascals (Pa) or N/m2.
- Fluid
- A liquid or a gas - something that can flow.
- Atmospheric pressure
- The pressure from the weight of air above you.
- Upthrust
- The upward force a fluid exerts on an object in it.
- Pascal
- One newton per square metre.
Learning cycle
Part 4 of 19
Learning cycle 1 of 2
Pressure on solid surfaces
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 5 of 19
Learn
Force spread over an area
Pressure = force / area. The same force over a smaller area gives a bigger pressure, which is why a sharp knife cuts and a blunt one does not, and why a drawing pin has a wide head to push and a fine point to pierce. Snowshoes and tractor tyres work the other way: the same weight over a much larger area, so the pressure is low enough not to sink.
Explore the idea
Part 6 of 19
Learn
Doing the calculation
Work in newtons and square metres to get pascals. A 600 N person standing on two feet of total area 0.03 m2 exerts 600 / 0.03 = 20 000 Pa. Balance on one foot and the area halves, so the pressure doubles. Watch the units: an area in cm2 must be converted before dividing, or the answer will be out by a factor of 10 000.
Quick check
Part 7 of 19
Quick check
Pressure is calculated as...
Quick check
Part 8 of 19
Quick check
A force of 300 N acts on 0.02 m2. The pressure is...
Reset break
Part 9 of 19
Pause
That's sitting 1 of 2 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Learning cycle
Part 10 of 19
Learning cycle 2 of 2
Pressure in fluids and in the air
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 11 of 19
Learn
Pressure in a liquid
In a fluid, pressure acts in every direction, not just downwards, and it increases with depth because more liquid is above you. That is why a dam is thickest at the base and why your ears hurt at the bottom of a pool but not in the shallow end. Pressure also depends on the liquid's density - it rises faster with depth in seawater than in fresh water.
Explore the idea
Part 12 of 19
Learn
Atmospheric pressure
Air has weight, so it presses on everything at roughly 100 000 Pa at sea level - about the weight of a small car spread over a desk. You do not feel it because it presses in from all sides and pressure inside your body matches it. Climb a mountain and there is less air above, so pressure falls; that is why a sealed crisp packet swells and why suction cups fail on a rough surface where air can leak back in.
Quick check
Part 13 of 19
Quick check
Snowshoes stop you sinking because they...
Quick check
Part 14 of 19
Quick check
Pressure in a liquid increases with depth because...
Common mix-ups
Part 15 of 19
Common mix-ups
Things people often get wrong
- Pressure in a fluid acts in all directions, not only downwards.
- A bigger area gives less pressure, not more.
- Air pressure is not zero at ground level - it is about 100 kPa.
- Areas in cm2 must be converted to m2 before calculating pascals.
Challenge round
Part 16 of 19
Game · Sort it
Which of these are true?
Drag each card into the right column. Tap a card first if dragging is fiddly.
True
Not true
Challenge round
Part 17 of 19
Game · Fill the gaps
Finish the sentences
Choose the word that belongs in each gap.
____ = force / area.
Air has weight, so it presses on everything at roughly 100 000 Pa at sea level - about the weight of a small car ____ over a desk.
Mastery quiz
Part 18 of 19
Marked quiz
End of lesson quiz: Pressure on surfaces, in fluids and in the atmosphere
6 questions, marked with the reasoning shown. No timer.
1. The unit of pressure is the...
2. A dam is thickest at the base because pressure...
3. Atmospheric pressure at sea level is roughly...
4. Standing on one foot instead of two...
5. A sealed packet swells up a mountain because outside pressure...
6. Which statement about fluid pressure is true?
Lesson round-up
Part 19 of 19
Lesson round-up
Ready when you are
Quiz score
Not sat
Games
Not played
Points this lesson
0
Best run
0 in a row
Luna: 0 out of 4 on the practice checks. Only if you feel up to it — one more?
Ask LunaDo the work
Your written work for this lesson
This is the part you'd do in your book at school. Pick the pieces below, write here or on paper, and photograph anything you draw or work out.
Why the school greenhouse failed
A science club grew tomato plants in the school greenhouse. In June the plants grew well: leaves were dark green and the fruit set quickly. In July the club went away for two weeks and left the greenhouse door shut. When they came back, the leaves were curled and pale and most of the fruit had dropped.
The club had a data logger inside. It showed the temperature reached 46 °C on several afternoons and the soil moisture fell close to zero. The club knew photosynthesis needs light, water and carbon dioxide, and that enzymes in a plant work best in a narrow range of temperature. Above that range the enzymes stop working properly.
The club repeated the summer with the door propped open and a water bottle drip on each pot. Temperature peaked at 31 °C and the plants survived.
Written for Good Learning Co.
Reading questions
- Name the variable the club changed the second time, and one variable they kept the same.
- Explain, using the words photosynthesis and enzymes, why the plants were damaged in July.
- Write a conclusion for the club's report, saying how far the evidence supports it.
Method and results
WritingWrite the aim, what you changed, what you measured and what you kept the same, then record your results in a table.
1 page + results table
Conclusion
WritingExplain what your results show and why, using the scientific words from the lesson. Say how you know you can trust them.
6–8 sentences
Labelled diagram
DiagramDraw the apparatus or the process in pencil, label it fully, and photograph it.
1 labelled diagram
Reading and notes
ReadingRead the lesson text again and write the key ideas in your own words.
Half a page of notes
Saved as you type.
Part 1 of 19 · Watch & discover
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Understand pressure in fluids and atmospheric pressure, and pressure on surfaces