GCSE-ready- Home
- Lessons
- KS3 Science
- Forces and their effects
Forces and their effects
🎬 The doodle video for this lesson is coming soon. Subscribe on YouTube to see it first.
A force is a push or a pull. Forces can start things moving, stop them, speed them up, slow them down or change their shape.
What is a force?
A force is an interaction between two objects: a push or a pull.
Forces are measured in newtons (N) using a newton meter.
Contact forces need the objects to touch: friction, air resistance, water resistance, a push.
Non-contact forces act across a gap: gravity, magnetic force, and the force between electric charges (static electricity).
Forces are measured in newtons (N) using a newton meter.
Contact forces need the objects to touch: friction, air resistance, water resistance, a push.
Non-contact forces act across a gap: gravity, magnetic force, and the force between electric charges (static electricity).
Force diagrams
We show a force with an arrow. The arrow points in the direction of the force, and a longer arrow means a bigger force.
To combine forces along one line: add forces in the same direction and subtract forces in opposite directions. The answer is the resultant force.
To combine forces along one line: add forces in the same direction and subtract forces in opposite directions. The answer is the resultant force.
Balanced and unbalanced
Balanced forces (resultant force zero): a still object stays still; a moving object keeps the same speed and direction.
Unbalanced forces: the object speeds up, slows down or changes direction, depending on which way the resultant force acts.
Forces can also change the shape of an object, by stretching or squashing it.
Unbalanced forces: the object speeds up, slows down or changes direction, depending on which way the resultant force acts.
Forces can also change the shape of an object, by stretching or squashing it.
A boat's engine pushes it forward with 900 N. Water resistance is 600 N backwards. Find the resultant force.
- The forces are in opposite directions, so subtract.
- 900 − 600
Answer: 300 N forwards
Forces are pushes or pulls measured in newtons. Add forces in the same direction and subtract opposite ones to find the resultant. Balanced: no change in motion. Unbalanced: speeds up, slows down or changes direction.
Check you have got it
Answer 6 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.