SingaporeScienceSyllabus dot point

Why does a sharp knife cut more easily than a blunt one, and how does pressure act in liquids and gases?

Define pressure as force divided by area, calculate pressure, and describe pressure in solids, liquids and gases with everyday examples

A practical answer to the N(T) Science point on pressure. How pressure depends on force and area, the pressure formula, and how pressure acts in solids, liquids and gases.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

This dot point wants you to know what pressure means, how to work it out from force and area, and how pressure acts in solids, liquids and gases. The big idea is that pressure is not just about how big a force is; it also depends on the area the force is spread over. The same force on a small area gives a high pressure, and on a large area gives a low pressure. You should be able to use the formula and explain everyday examples.

The answer

What pressure means

Pressure tells you how concentrated a force is on a surface. It depends on two things: the size of the force, and the area it is pushing on. The same force feels much sharper when it is pressed onto a tiny area.

Pressure is measured in pascals (Pa). One pascal is one newton of force pushing on one square metre.

The pressure formula

Pressure is worked out with this formula:

\text{pressure} = \frac{\text{force}}{\text{area}}

The force is in newtons (N) and the area is in square metres (m squared), giving pressure in pascals (Pa). For example, a force of 100 N on an area of 2 square metres gives a pressure of $100 \div 2 = 50\ \text{Pa}$ .

Pressure in solids

For solids, the key idea is area. The same force on a smaller area gives a higher pressure.

This is why a sharp knife cuts better than a blunt one: the sharp edge has a tiny area, so even a small force gives a very high pressure that cuts through. It is also why camels and snowshoes have wide feet: a large area spreads the weight out, giving a low pressure so they do not sink into sand or snow. Drawing pins are sharp at the point (high pressure to go in) and flat at the top (low pressure on your thumb).

Pressure in liquids

In a liquid, pressure pushes in all directions, not just down. The pressure in a liquid gets greater the deeper you go, because there is more liquid above pushing down.

This is why a dam wall is built thicker at the bottom, where the water pressure is highest, and why a diver feels their ears squeezed more the deeper they swim. Water also squirts out faster from a hole near the bottom of a tall container than from a hole near the top.

Pressure in gases

Gases also push out in all directions and exert pressure. The fast-moving particles of a gas hit the walls of their container and push on them.

This is why a blown-up balloon or a car tyre stays firm: the gas inside pushes out evenly on every part of the wall. Squashing a gas into a smaller space pushes the particles closer together so they hit the walls more often, which increases the pressure. The air around us, the atmosphere, also presses on everything; this is air pressure.

Worked example: comparing pressure under two shoes

A person weighs $600\ \text{N}$ . They first stand flat on both shoes, with a total area of $0.03\ \text{m}^2$ touching the ground. Then they balance on one thin heel with an area of only $0.001\ \text{m}^2$ . Compare the pressure in each case.

Step 1: Work out the pressure standing flat

Use pressure = force $\div$ area. The force is the person's weight, $600\ \text{N}$ , and the area is $0.03\ \text{m}^2$ . Pressure = $600 \div 0.03 = 20\,000\ \text{Pa}$ .

Step 2: Work out the pressure on one heel

The force is still $600\ \text{N}$ , but now the area is only $0.001\ \text{m}^2$ . Pressure = $600 \div 0.001 = 600\,000\ \text{Pa}$ .

Step 3: Compare the two answers

Standing flat gives $20\,000\ \text{Pa}$ ; balancing on the thin heel gives $600\,000\ \text{Pa}$ . The pressure on the heel is much higher, even though the force (the weight) is exactly the same.

Step 4: Explain why

The weight did not change; only the area did. A much smaller area concentrates the same force, so the pressure is far greater. This is why a thin heel can dent a soft floor while a flat shoe does not. Area is the key.

Examples in context

Example 1. A tractor in a muddy field. A heavy tractor does not sink into soft mud because it has very wide tyres. The wide tyres spread the tractor's large weight over a big area, giving a low pressure on the ground. A narrow car wheel carrying the same weight would have a much higher pressure and would sink in.

Example 2. Drinking through a straw. When you suck on a straw, you lower the air pressure inside it. The higher air pressure pushing down on the drink outside the straw then forces the liquid up into your mouth. This everyday action works because gases (here, the air) exert pressure on everything around them.

Try this

Cue. A force of $80\ \text{N}$ pushes on an area of $4\ \text{m}^2$ . Work out the pressure. Pressure = force $\div$ area = $80 \div 4 = 20\ \text{Pa}$ .
Cue. Explain why a sharp knife cuts food more easily than a blunt knife. The sharp edge has a much smaller area, so the same force gives a much higher pressure, which cuts more easily.
Cue. State what happens to the pressure on a submarine as it dives deeper, and why. The pressure increases, because there is more water above the submarine pushing down on it the deeper it goes.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksA box rests on the floor. The box pushes down with a force of

200\ \text{N}

and its base has an area of

0.5\ \text{m}^2

. (a) Write the formula for pressure. (b) Calculate the pressure on the floor. (c) State the unit. (d) State what happens to the pressure if the same box is stood on a smaller side.

Show worked answer →

(a) Pressure = force $\div$ area.

(b) Pressure = $200 \div 0.5 = 400$ . So the pressure is $400\ \text{Pa}$ (pascals).

(d) Standing the box on a smaller side puts the same force on a smaller area, so the pressure increases (gets bigger).

What markers reward: stating pressure = force $\div$ area, doing the division to get 400, giving the unit pascal (Pa), and knowing that a smaller area with the same force gives a higher pressure.

Original3 marksA diver swims deeper and deeper in the sea. (a) State what happens to the water pressure on the diver as they go deeper. (b) Explain why this happens. (c) Give one everyday example that shows a gas exerts pressure.

Show worked answer →

(a) The water pressure on the diver increases as they go deeper.

(b) The deeper the diver goes, the more water there is above them pushing down, so there is a greater weight of water pressing on them, which increases the pressure.

(c) Any sensible example, such as: a blown-up balloon or tyre stays firm because the gas inside pushes out on the walls, or air pressure holds up a suction cup on a window.

What markers reward: stating that pressure increases with depth, explaining it as more water above pressing down, and a clear example of a gas exerting pressure (balloon, tyre or similar).