What is the difference between mass and weight in O-Level Physics?

Mass is the amount of matter in an object, measured in kilograms, and it is the same everywhere. Weight is the gravitational force on that mass, measured in newtons, and it changes with the gravitational field strength, so an object weighs less on the Moon than on Earth even though its mass is unchanged. The link is $W = mg$.

How do you calculate density and what are its units?

Density is mass per unit volume, $\rho = m/V$, measured in $\text{kg m}^{-3}$ (or $\text{g cm}^{-3}$). To measure it you find the mass on a balance and the volume either from dimensions for a regular solid or by displacement of water for an irregular one, then divide. An object floats in a liquid if its density is less than that of the liquid.

Why does pressure in a liquid increase with depth?

Pressure in a liquid increases with depth because a deeper point has more liquid above it, so the weight of that column pressing down is greater. The pressure due to a liquid column is $p = h \rho g$, where $h$ is the depth, $\rho$ the density and $g$ the gravitational field strength. It does not depend on the shape or width of the container, only on depth.

How does a mercury barometer measure atmospheric pressure?

A mercury barometer measures atmospheric pressure by balancing it against the weight of a mercury column in an inverted tube closed at the top. Atmospheric pressure pushes the mercury up until the column's pressure $h \rho g$ equals the atmospheric pressure; standard atmospheric pressure supports a column about $760\ \text{mm}$ of mercury high. A taller column means higher atmospheric pressure.

How is pressure in a solid calculated and why do sharp objects cut?

Pressure on a solid surface is force per unit area, $p = F/A$, measured in pascals ($\text{Pa} = \text{N m}^{-2}$). For a fixed force, a smaller area gives a larger pressure, which is why a sharp knife or a drawing pin concentrates the force on a tiny area and cuts or pierces easily, while a flat ski spreads weight over a large area to reduce pressure on snow.

SingaporePhysics

Mass, Weight, Density and Pressure for Singapore O-Level Physics (6091): the difference between mass and weight, calculating density, pressure in solids and liquids, and gas pressure with the mercury barometer

A Singapore O-Level Physics (SEAB 6091) overview of Mass, Weight, Density and Pressure. It covers the difference between mass and weight, how to measure and calculate density, pressure in solids and the pressure in liquids that increases with depth, and gas pressure measured with the mercury barometer and manometer.

Generated by Claude Opus 4.87 min readSEAB-6091Updated 2026-06-13

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

Jump to a section

What this module covers
Mass and weight
Density
Pressure in solids and liquids
Gas pressure and the mercury barometer
How this module is examined
Check your knowledge

What this module covers

Mass, Weight, Density and Pressure draws together several properties of matter that are easy to confuse, so O-Level Physics (SEAB 6091) treats them carefully. The module separates mass from weight, defines density as a property of a material, and develops pressure first for solids and then for liquids and gases, where it varies with depth and supports the mercury barometer.

These ideas connect back to Forces and Dynamics (weight is a force) and forward to Thermal Physics, where gas pressure links to particle motion. Each dot point below gives full worked answers and practice.

Mass and weight

Mass and weight draws the central distinction: mass is the quantity of matter in kilograms and is constant, while weight is the gravitational force in newtons and depends on location through

W = mg,

where

g

is the gravitational field strength (about

9.8\ \text{N kg}^{-1}

near Earth, but smaller on the Moon).

Keeping the units straight, kilograms for mass and newtons for weight, is the first thing examiners check.

Density

Density defines density as mass per unit volume,

\rho = \frac{m}{V},

and shows how to measure the volume of a regular solid from its dimensions or of an irregular solid by water displacement. Density decides whether an object floats: it floats if its density is less than that of the liquid.

Pressure in solids and liquids

Pressure and pressure in liquids defines pressure as force per unit area,

p = \frac{F}{A},

measured in pascals. It explains why a small area gives a large pressure (sharp knives, drawing pins) and develops the pressure due to a liquid column,

p = h \rho g,

which increases with depth and does not depend on the container's shape.

Gas pressure and the mercury barometer

Gas pressure and the mercury barometer extends pressure to gases. Atmospheric pressure is measured by a mercury barometer, in which the atmosphere supports a mercury column about $760\ \text{mm}$ high at standard conditions. A manometer compares a gas pressure with atmospheric pressure using the height difference of a liquid in a U-tube.

How this module is examined

Name the quantity and its unit. Mass in kilograms, weight in newtons, density in $\text{kg m}^{-3}$ , pressure in pascals.
Choose the right pressure formula. Use $p = F/A$ for a solid surface and $p = h \rho g$ for a liquid column.
Use displacement for irregular solids. Find volume by the rise in water level, then divide mass by volume for density.

Check your knowledge

Recall and calculation questions across the module. Try them, then check the worked solutions.

State the difference between the mass and the weight of an object. (2 marks)
A block has a mass of $600\ \text{g}$ and a volume of $200\ \text{cm}^3$ . Calculate its density. (2 marks)
A force of $50\ \text{N}$ acts on an area of $0.25\ \text{m}^2$ . Calculate the pressure. (2 marks)
State why pressure in a liquid increases with depth. (1 mark)
Taking $g = 9.8\ \text{N kg}^{-1}$ , calculate the weight of a $5.0\ \text{kg}$ bag. (2 marks)

Sources & how we know this

Singapore-Cambridge GCE O-Level Physics (Syllabus 6091) — Singapore Examinations and Assessment Board (2026)