Do I need a lab to practise?

No, the Paper 3 graph and analysis skills can be practised from home with example data.

Why does the membrane point in the same direction for the main readings?

To make it a fair test, only the depth should change. Keeping the membrane facing the same way means the responding variable H depends only on the depth. Turning it later, at one fixed depth, is a separate check that pressure acts equally in all directions.

Does the width of the vessel affect the pressure?

No. The pressure at a point in a liquid depends only on the depth, the density and g, not on the shape or width of the container. That is why P = hρg contains no area or volume term.

Why is the graph a straight line through the origin?

Because P = hρg, and ρ and g are constant, the pressure is a fixed multiple of the depth. At the surface the depth is zero, so the extra pressure is zero, which is why the line passes through the origin.

Experiment: How the depth of a liquid affects the pressure

Aim

To investigate the relationship between the depth below the surface of a liquid and the pressure at that depth.

Variables

Manipulated: Depth below the liquid surface, h
Responding: Pressure, shown by the height difference in the manometer, H
Constant: The same liquid (same density)

Apparatus & materials

Tall vessel or measuring cylinder of water
Thistle funnel with a thin rubber membrane
U-tube manometer
Rubber tubing
Ruler
Retort stand

Procedure

Fill the tall vessel with water and connect the membrane-covered thistle funnel to the U-tube manometer with rubber tubing.
Lower the funnel so its membrane is at a depth h = 5.0 cm below the water surface.
Record the height difference H between the two liquid levels of the manometer.
Keep the membrane facing the same direction and increase the depth to h = 10.0, 15.0, 20.0 and 25.0 cm.
Record the manometer height difference H at each depth.

Tabulating results

Record the depth h in cm and the manometer height difference H in cm (which represents the pressure). Keep the decimal places consistent.

The graph

Plot H (pressure, y-axis) against depth h (x-axis). A straight line through the origin shows the pressure is directly proportional to the depth.

Analysis

The pressure in a liquid is given by P = hρg, so it increases in direct proportion to the depth. Turning the membrane to face different directions at the same depth shows the pressure acts equally in all directions.

Precautions

Measure the depth to the centre of the membrane.
Read both manometer levels at eye level to avoid parallax.
Make sure there is no air leak in the tubing.

Pressure · Graph skills

Sample results and what they show

Example data, taking the water manometer height difference H as a measure of the pressure at the membrane:

Depth h = 5.0 cm, manometer difference H = 4.5 cm
h = 10.0 cm, H = 9.0 cm
h = 15.0 cm, H = 13.5 cm
h = 20.0 cm, H = 18.0 cm
h = 25.0 cm, H = 22.5 cm

As the depth doubles, the manometer difference doubles too, so the pressure is a constant multiple of the depth. This shows that the pressure in a liquid increases in direct proportion to the depth below the surface.

If you turn the membrane to face sideways, upwards and downwards at the same depth, H stays the same, which shows the pressure at a point acts equally in all directions.

Reading the graph and finding the answer

Plot the manometer difference H (pressure, y-axis) against depth h (x-axis). The points give a straight line through the origin, confirming that pressure is directly proportional to depth.

Find the gradient with a large triangle on the best-fit line, using (5.0 cm, 4.5 cm) and (25.0 cm, 22.5 cm):

gradient = ΔH ÷ Δh = (22.5 − 4.5) cm ÷ (25.0 − 5.0) cm = 18.0 cm ÷ 20.0 cm = 0.90 (no unit)

The gradient has no unit because both axes are lengths. It is constant, which is what confirms the proportional relationship P = hρg: the pressure depends only on the depth h, the liquid density ρ and g, all of which are fixed here except the depth.

A line through the origin means zero extra pressure at the surface.

Marks examiners look for

For the practical marks: measure the depth h to the centre of the membrane each time, read both manometer levels at eye level to avoid parallax, and check that the rubber tubing has no air leaks before starting.

For the Paper 3 science-process-skill marks, identify the variables: the manipulated variable is the depth h, the responding variable is the pressure shown by the manometer difference H, and the fixed variable is the same liquid, so the density stays constant. Tabulate h and H in cm with a heading and unit for each column and consistent decimal places.

Plot on even scales, draw one thin best-fit line, and show the gradient triangle with its coordinates. The conclusion should state that the pressure is directly proportional to the depth, and you can add that turning the membrane shows the pressure acts equally in all directions at a given depth.

Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)

Written by the spmphysics.com.my editorial team.· Updated 5 Sept 2026