The unit tells the quantity

A unit tells you what kind of measurement a number represents. The number 5 could mean 5 kg, 5 m, 5 s, 5 N, or something else entirely; the unit gives the number its physics meaning.

In this cluster, Pearson expects you to use these units:

The safest exam habit is to read the quantity first, then choose the unit. For example, a mass is measured in kg, but a force such as weight is measured in N. Those two are often confused because everyday speech says "weighs 50 kg", but in physics kg is mass, not force.

Kilogram, metre and second

The simplest units in this lesson are kg, m, and s.

These units are the building blocks for the compound units you meet later. For example, a speed in m/s is built from metres and seconds. An acceleration in m/s^2 is also built from metres and seconds, but it has a different meaning because the seconds appear twice.

Use the exact symbol. Write kg, not Kg or kgs; write m, not M; write s, not sec unless the question has already accepted that wording. In most Edexcel calculations, a correct number with a missing or wrong final unit can lose a mark.

Units for motion

Two motion units in this lesson are made from metres and seconds.

Speed and velocity use m/s because they compare distance with time. A speed of 4 m/s means the object covers 4 metres each second, if the speed is steady.

Acceleration uses m/s^2 because it is about how velocity changes each second. An acceleration of 3 m/s^2 means the velocity changes by 3 m/s every second. The extra "per second" is why acceleration has the squared second in the unit.

Do not shorten m/s^2 to m/s. That changes the quantity from acceleration to speed or velocity. In the same way, do not use m/s^2 for a speed just because the object is moving quickly.

Newtons and newtons per kilogram

The unit of force is the newton, symbol N. Forces can change an object's speed, direction, or shape, but in this lesson the key point is the unit: if the quantity is a force, its unit is N.

The unit N/kg means newtons per kilogram. It is used for gravitational field strength: the force due to gravity on each kilogram of mass. For example, a gravitational field strength of 10 N/kg means each kilogram of mass experiences a gravitational force of about 10 N.

The slash in N/kg matters. It means "per kilogram". N/kg is not the same as N, and it is not the same as kg. A common mistake is to write a gravitational field strength as 10 N, but 10 N is a force, not force per kilogram.

Keep these pairs separate:

Separate Physics: Nm and kg m/s

Separate Physics students also need two extra units in this cluster: Nm and kg m/s.

Nm is newton metre. It is used for the moment of a force, which is the turning effect of a force about a pivot. The unit combines a force unit, N, with a distance unit, m. At this stage, you only need the unit and its meaning; later work covers how moments are calculated and balanced.

kg m/s is kilogram metre per second. It is used for momentum. The unit combines mass, kg, with velocity, m/s. At this stage, you only need to recognise and use the unit; later work covers momentum calculations and changes in momentum.

These two units look similar to other units in the course, so read them carefully:

Exam unit habits

Units are quick marks when you handle them carefully. Before you write a final answer, check three things:

1. Does the unit match the quantity asked for?
2. Does the symbol use the correct capital letters, such as N for newton?
3. Have you kept the full compound unit, such as m/s^2, N/kg, or kg m/s?

Here are some typical corrections:

A good check is to say the unit out loud in full: "newtons per kilogram" is not the same as "newtons"; "kilogram metre per second" is not the same as "metres per second"; "metres per second squared" is not the same as "metres per second".