GCSE-ready- Home
- Lessons
- GCSE & IGCSE Physics
- Getting started: forces, mass and weight
Getting started: forces, mass and weight
🎬 The doodle video for this lesson is coming soon. Subscribe on YouTube to see it first.
Forces explain why things start, stop, speed up and change direction. GCSE Physics builds Newton's laws on these KS3 ideas.
What is a force?
A force is a push or a pull, measured in newtons (N) with a newtonmeter (force meter). Forces are drawn as arrows: the length shows the size, the arrow shows the direction.
Contact forces: friction, air resistance, normal contact force (reaction), tension, upthrust.
Non-contact forces: weight (gravity), magnetic force, electrostatic force.
Contact forces: friction, air resistance, normal contact force (reaction), tension, upthrust.
Non-contact forces: weight (gravity), magnetic force, electrostatic force.
Balanced and unbalanced
The resultant force is the single force that has the same effect as all the forces together. Add forces in the same direction; subtract opposite ones.
Balanced (resultant = 0): a still object stays still; a moving object keeps the same speed and direction.
Unbalanced: the object speeds up, slows down or changes direction.
Balanced (resultant = 0): a still object stays still; a moving object keeps the same speed and direction.
Unbalanced: the object speeds up, slows down or changes direction.
Mass and weight
Mass is the amount of matter, in kg. It is the same everywhere.
Weight is the force of gravity on a mass, in N.
weight = mass × gravitational field strength
On Earth use g = 10 N/kg. On the Moon g is about 1.6 N/kg, so you weigh less there, but your mass is unchanged.
Weight is the force of gravity on a mass, in N.
weight = mass × gravitational field strength
On Earth use g = 10 N/kg. On the Moon g is about 1.6 N/kg, so you weigh less there, but your mass is unchanged.
What is the weight of a 55 kg student on Earth? Use g = 10 N/kg.
- weight = mass × g
- = 55 × 10
Answer: 550 N
Forces are in newtons. Balanced forces: no change in motion. Unbalanced: speeding up, slowing down or turning. Weight (N) = mass (kg) × g; mass never changes from place to place.
Check you have got it
Answer 8 quick questions with instant marking. If you get one wrong, GCSE-ready shows you why and gives you another go. It is free, and you do not need an account.