Forces · Grade 9
A force is a push or a pull, measured in newtons (N). Forces have a size and a direction, so we draw them as arrows. Add up all the forces to find the resultant: if it is zero the object keeps doing what it was doing; if not, it speeds up, slows down or changes direction.
Key facts
- Contact forces: friction, air resistance, upthrust, tension, normal contact force. Non-contact: gravity, magnetic, electrostatic.
- Weight is the force of gravity on an object; it acts downwards.
- Resultant force: add forces in the same direction, subtract opposite ones.
- Balanced forces (resultant zero): stays still, or moves at a steady speed in a straight line.
- Unbalanced forces: the object accelerates in the direction of the resultant.
- Forces at right angles combine by Pythagoras.