What is a force?

A force is an action capable of:

• setting an object in motion, or bringing it to a halt
• changing its path
• deforming it

It is measured in newtons (N).

A force has a direction

Pushing a door to the left or to the right with the same magnitude does not produce the same effect at all. A force is therefore not simply a number: it also has a direction and a sense.

This is why it is represented by an arrow, rather than simply by a number. The length of the arrow indicates the magnitude, whilst its orientation indicates the direction.

Mass and weight: the most common confusion

	Mass	Weight
What it is	a quantity of matter	a force of attraction
Unit	kilogram (kg)	newton (N)
On the Moon	the same	divided by 6
Instrument	balance	dynamometer

Weight is calculated as follows:

P = m x g

where g ≈ 9,8 N/kg is the value on Earth.

An object weighing 5 kg therefore has a weight of 5 x 9,8 = 49 N.

The same object taken to the Moon (g ≈ 1,6 N/kg) retains the same mass of 5 kg, but its weight drops to 8 N. This is why astronauts bounce: their mass remains unchanged, but the force holding them to the ground is six times weaker.

Saying ‘I weigh 70 kilos’ is therefore a misnomer: one has a mass of 70 kg and a weight of approximately 686 N. Everyone says this, including physicists — but in practice, the distinction is noted.

Forces that cancel each other out

A book resting on a table is subject to two forces:

• its weight, acting downwards
• the table’s reaction force, acting upwards

They have the same magnitude and opposite directions: they cancel each other out. The book does not move — not because there is no force, but because the sum of the forces is zero. The rest will be covered in the next lesson.