Pressure in a liquid comes from the weight of the liquid above pushing down on everything below. Go deeper and there is a taller column of water overhead, so the push is greater. The pressure depends on the depth, the density of the liquid and gravity, but not on the width or shape of the container.
You feel this when diving to the bottom of a swimming pool: your ears begin to hurt because the water presses in on your eardrums more strongly the deeper you go. A dam is built much thicker at its base than at the top for the same reason, since the water pushes hardest near the bottom.
A kampung well shows it too: water spurts out faster and further from a hole near the base of a tall water tank than from one near the top.
In SPM you should relate liquid pressure to depth, density and gravity, and explain why it does not depend on container shape.
Common misconceptions
- A wider container holds water at higher pressure -> Pressure depends on depth, not width; a narrow deep column can have higher pressure than a wide shallow one.
- Pressure only pushes downward -> Liquid pressure acts in all directions at a given depth, including sideways and upward.
- Air above the water does not add to the pressure -> Atmospheric pressure pushes on the surface and adds to the pressure felt below it.
The physics behind it
The pressure due to a liquid at a depth h is given by P = ρgh, where ρ is the density of the liquid in kilograms per cubic metre, g is the gravitational field strength in newtons per kilogram, and h is the vertical depth in metres. The result is a pressure in pascals, where one pascal is one newton per square metre.
The formula follows from the weight of the column of liquid resting on each square metre, so only the vertical depth matters, not the width or shape of the container.
For a diver 15 m below the surface of fresh water, the pressure from the water alone is P = ρgh = 1000 kg m⁻³ × 9.81 N kg⁻¹ × 15 m = 147150 Pa, roughly one and a half times normal atmospheric pressure. Go twice as deep and the pressure from the water doubles, because P is directly proportional to h.
This pressure acts equally in all directions at that depth, pressing inward on a diver's mask and body from every side, not just downward.
In a Malaysian home
Malaysian homes show this every day through the water tank on the roof. Water stored high in the tank presses down through the pipes, and the greater the height of water above a tap, the greater the pressure that drives the flow.
A tap on the ground floor, further below the tank, usually runs harder than one on the upper floor, which sits closer to the tank and so has a smaller height of water above it.
That is why the top-floor shower can feel weak when the tank level drops, and why some houses fit a pump to boost it. The same reasoning explains why the tank is placed as high as possible: raising the water increases h in P = ρgh and so increases the pressure at every outlet below.
A garden hose fed from a higher tank sprays further than one fed from a low bucket. In each case it is the vertical height of water above the opening, not the amount stored or the width of the tank, that sets the pressure.
How this comes up in SPM
In Paper 2 this belongs to the Pressure chapter and is examined with command words such as explain, relate, describe and calculate. You might be asked to relate liquid pressure to depth, density and gravitational field strength, to explain why a dam wall is thicker at the base, or to calculate the pressure at a stated depth using P = ρgh.
Questions may also ask you to describe an observation, such as water jets travelling different distances from holes at different heights.
The idea connects to its neighbours in the same chapter: pressure in solids given by P = F/A, which introduces the pascal, atmospheric pressure pushing down on the liquid surface and adding to the total, and the manometer and barometer that measure pressure using columns of liquid. It also underpins Pascal's principle and Archimedes' principle, which come later in the chapter.
When you tackle a depth question, be clear whether you are asked for the pressure due to the liquid alone or the total pressure including the atmosphere above.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)