What Is a Force?
A force is a push or a pull that acts on an object. Forces can change an object's speed, direction, or shape.
Forces are everywhere — when you kick a football, carry a bag, or even just stand still, forces are acting on you and the objects around you.
In physics, we describe forces using two key properties:
- Magnitude — how big the force is, measured in Newtons (N)
- Direction — which way the force is acting
Because forces have both magnitude AND direction, they are called vector quantities. This means we represent them as arrows in diagrams — the length of the arrow shows the size, and the arrowhead shows the direction.
The unit of force is the Newton (N), named after Sir Isaac Newton. A force of 1 N is roughly the weight of a small apple!
Weight — The Gravitational Force
Weight is the gravitational force acting on an object due to a nearby massive body (like a planet). It always acts downward toward the centre of that body.
Weight is calculated using the equation:
Where:
- = weight in Newtons (N)
- = mass in kilograms (kg)
- = gravitational field strength in N/kg (on Earth, )
Mass and weight are NOT the same thing!
- Mass is the amount of matter in an object — it never changes, no matter where you are.
- Weight depends on the gravitational field strength, so it changes on different planets.
A person with a mass of 60 kg has the same mass on the Moon as on Earth, but their weight is very different!
Calculating Weight on Earth and the Moon
A student has a mass of 50 kg. Calculate their weight on Earth ( N/kg) and on the Moon ( N/kg).
On Earth:
On the Moon:
The student's mass is still 50 kg in both places, but their weight is much less on the Moon because gravity is weaker there.
Think of mass as the number of coins in your wallet — it doesn't matter where you go, the number stays the same. Weight is like the value of those coins — it changes depending on which country (or planet!) you're in.
10 more sections in this topic
Pick this up in your Library: it holds the whole topic, notes, cheatsheet and questions.