Pressure on surfaces and in fluids
The same force can crush or barely mark a surface, depending on the area it acts over. Pressure explains snowshoes, drawing pins, submarines and the weather.
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
- • Pressure on a surface
- • Pressure in liquids
- • Atmospheric pressure
- • Upthrust and floating
This lesson at a glance
- 28 minutes
- 21 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 21
Visual introductionPicture this
Pressure on surfaces and in fluids
The same force can crush or barely mark a surface, depending on the area it acts over. Pressure explains snowshoes, drawing pins, submarines and the weather.
In a nutshell
Understand pressure in fluids and atmospheric pressure, and pressure on surfaces
Learning cycle
Part 2 of 21
Learning cycle 1 of 2
Part 1 · Pressure on a surface
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 3 of 21
Learn
Pressure on a surface
Pressure = force ÷ area, measured in pascals, where one pascal is one newton per square metre. A drawing pin concentrates a small force onto a tiny point, creating enormous pressure, while snowshoes spread weight over a large area so the pressure stays below what the snow can support.
Reset break
Part 4 of 21
Pause
That's sitting 1 of 5 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.
Explore the idea
Part 5 of 21
Learn
Pressure in liquids
In a liquid, pressure acts in all directions and increases with depth, because a greater weight of liquid sits above. Denser liquids exert more pressure at the same depth. This is why dam walls are thicker at the base and why deep-sea vehicles need reinforced hulls.
Quick check
Part 6 of 21
Quick check
Part 7 of 21
Reset break
Part 8 of 21
Pause
That's sitting 2 of 5 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 9 of 21
Learning cycle 2 of 2
Part 2 · Atmospheric pressure
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 10 of 21
Learn
Atmospheric pressure
The atmosphere presses on everything at roughly 100,000 Pa at sea level, caused by the weight of air above. Pressure falls with altitude because there is less air above you and the air is less dense, which is why aircraft cabins are pressurised.
Explore the idea
Part 11 of 21
Learn
Upthrust and floating
Because pressure increases with depth, the upward force on the bottom of a submerged object is greater than the downward force on its top. The difference is upthrust. An object floats when upthrust equals its weight, which is why a steel ship shaped to displace a large volume of water floats while a steel block sinks.
Reset break
Part 12 of 21
Pause
That's sitting 3 of 5 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.
Quick check
Part 13 of 21
Quick check
Part 14 of 21
Explore the idea
Part 15 of 21
Worked example
Worked answer: a 600 N person stands on one foot of area 0.015 m². Find the pressure (3 marks)
Pressure = force ÷ area (1). Pressure = 600 ÷ 0.015 (1) = 40,000 Pa, or 40 kPa (1). Standing on both feet doubles the area and therefore halves the pressure to 20 kPa, while the force, their weight, stays the same.
Reset break
Part 16 of 21
Pause
That's sitting 4 of 5 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.
Challenge round
Part 17 of 21
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 18 of 21
Game · Fill the gaps
Finish the sentences
Choose the word that belongs in each gap.
____ = force ÷ area, measured in pascals, where one pascal is one newton per square metre.
Challenge round
Part 19 of 21
Game · Recall cards
A cheetah covers 120 m in 4 s. Its speed is…
Card 1 of 4
Mastery quiz
Part 20 of 21
Marked quiz
End of lesson quiz: Pressure on surfaces and in fluids
4 questions, marked with the reasoning shown. No timer.
1. A cheetah covers 120 m in 4 s. Its speed is…
2. A flat line on a distance–time graph means…
3. Reducing area with the same force…
4. Pressure is calculated as…
Lesson round-up
Part 21 of 21
Lesson round-up
Ready when you are
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Ask LunaPart 1 of 21 · Watch & discover
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Understand pressure in fluids and atmospheric pressure, and pressure on surfaces