KS3 · Physics

Pressure at different depths and heights

Swim one metre down and the pressure changes by about as much as climbing 1,000 metres through air. Why are they so different?

Physics · Pressure in fluids

One journey: from thin air to deep water

Step down through the zones, top to bottom. The zones are drawn the same size so you can click through them, but the spacing is NOT to scale — read the pressure numbers, not the gaps. (1 kN = 1,000 N, so 100 kN/m² is 100,000 N/m².)

↑ Thin air, high up

↓ Deep water

Zone 1 of 7

10 km up

Up here there is very little air above you, so very little weight pressing down. The pressure is only about 26 kN/m² (about 26,000 N/m²).

  • Very little air above you
  • The atmosphere reaches up to about 100 km, with no exact edge — the air just gets thinner and thinner

More fluid above you means more pressure. The rule is the same in air and in water — but water is so dense that its pressure changes far faster.

Physics · Air versus water

How far is 'one metre of water' in air?

Going down 1 metre in water changes the pressure by about 10,000 N/m². About how many metres would you have to climb up through air to change the pressure by the same amount?

Your estimate

1000m

0m2000m

Physics · Fluid pressure

What is the water really doing to the diver?

A diver hangs in the sea, well below the surface.

Which of these is closest to what you think right now?
How sure are you?

Dams

?

Reason it through

Why is a dam thicker at the bottom than at the top?

Link 1 of 4

First link · your turn

Think about the water held back by the dam. How does the amount of water above a point change as you go down the dam wall?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Work it out

Water pushes up on the bottom of a raft with an upthrust of 600 N. That force is spread over 2 m² of the raft's bottom. Find the pressure, using pressure = force ÷ area.

Finding pressure (Pa)

p = F ÷ A

Answer

p = 600 ÷ 2

= 300 Pa

Predict, then check

Two sealed containers full of air. A vacuum pump removes all the air from inside a metal can. Separately, a sealed weather balloon rises high up into the atmosphere (an unopened crisp packet on a plane does the same).

What happens to the can, and what happens to the balloon?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

One question explains it all: how much fluid is on top of you?

What you need to know

  • Pressure in a fluid comes from the weight of all the fluid above you.
  • In water, more depth means more water above you, so greater pressure. In air, more height means less air above you, so less pressure.
  • Objects at the same depth have the same pressure on them, and it acts from all directions.
  • Water is much denser than air, so pressure changes far faster with depth in water than with height in air.

The big picture

Pressure in a fluid comes from the weight of all the fluid above you. Dive deeper into water and there is more water above you, so the pressure grows quickly. Climb higher through air and there is less air above you, so the pressure falls slowly. Pressure is also a force spread over an area, and differences in pressure explain dams, crushed cans and swelling balloons.

Key points

1Water pressure is about 100,000 N/m² near the surface and rises by about 10,000 N/m² for every metre of depth, so about 10 m down doubles it.
2Atmospheric pressure is about 100,000 N/m² at sea level and falls with height: about 34 kN/m² at about 9 km and about 26 kN/m² at 10 km.
3Pressure = force ÷ area, and force = pressure × area. Pressure is measured in N/m² (pascals, Pa).
4Upthrust is an upwards force due to pressure. A bigger area on the bottom of a raft allows a bigger upthrust.
5A pressure difference between inside and outside explains a crushed can and an expanding balloon.

Worked example

Problem

A small raft has a flat bottom with an area of 3 m². Suppose the water pushes up on the bottom with a pressure of 400 N/m² (friendly made-up numbers). What upthrust does that give? Then: what happens to the maximum upthrust if the raft's bottom area doubles to 6 m²?

⚠ Watch out

Thinking water pressure only pushes down on you, or that it depends on how much water is around you. In fact it acts from every direction and depends on how much water is above you: the same at the same depth, greater deeper down.

🧠

Memory hook

How much is on top of you? Deeper in water means more on top, so more pressure. Higher in air means less on top, so less pressure.

✓

Check yourself

Which has the greater pressure on it: a diver 5 m under the sea, or a hiker 3 km up a mountain? Say why. Answer: the diver, because there is far more fluid above.

Flashcards

(14)
What is the equation for pressure?
pressure = force ÷ area. Measured in N/m² (pascals, Pa).
How do you find the force produced by a pressure?
Rearrange pressure = force ÷ area: force = pressure × area.
What causes the pressure beneath the sea?
The weight of all the water above you.
How does water pressure change with depth?
It increases. Deeper means more water above you and a greater pressure.
About how much does water pressure rise per metre of depth?
About 10,000 N/m² for every metre, starting from about 100,000 N/m² near the surface.
What happens to the pressure about 10 m under water?
It doubles, because 10 m adds about 100,000 N/m² to the 100,000 N/m² at the surface.
What is atmospheric pressure, and why does it exist?
The pressure caused by the atmosphere. Air has weight, so it pushes down and acts on all objects inside it.
How does atmospheric pressure change as you go higher?
It decreases, because there is less air above you. About 100 kN/m² at sea level, about 34 kN/m² at about 9 km.
Why does air pressure change so much more gradually than water pressure?
Air is much less dense than water. About 1,000 m of air changes the pressure by about as much as about 1 m of water.
Two objects are at the same depth in water. How do the pressures on them compare?
The same, because the weight of water above them is the same.
In which directions does pressure act on a submerged object?
All directions: top, bottom, sides, front and back.
What is upthrust?
A force acting upwards due to pressure, such as the water pushing up on the bottom of a raft.
Why can a bigger raft bottom give a bigger upthrust?
The pressure at the water's surface is constant, so a larger area gives a larger force (force = pressure × area).
Why does a vacuum pump crush a can?
Removing the air from inside leaves the large air pressure outside unbalanced, so it pushes the can in.

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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