Forces · Grade 4
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.
Worked examples
Example 1
A 70 kg skydiver falls at a steady speed. What is the air resistance on her? (Take g = 9.8 N/kg.)
Answer: 686 N upwards
Example 2
Two ropes pull a boat with forces of 300 N east and 400 N north. Find the resultant force.
Answer: 500 N, at 53° north of east