Forces

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. When the forces on something are balanced it keeps doing what it was doing; when they are unbalanced it speeds up, slows down or changes direction.

Key facts

  • Forces are measured in newtons (N) with a newton meter.
  • Resultant force: add forces in the same direction, subtract opposite ones.
  • Balanced forces (resultant 0): still, or steady speed in a straight line.
  • Unbalanced: it accelerates in the direction of the resultant force.
  • Friction and air resistance act against motion.

Worked examples

Example 1

A box is pushed right with 30 N while friction pulls left with 12 N. Find the resultant force.

  1. The forces point opposite ways, so take the smaller from the bigger. 30 − 12 = 18
  2. It acts in the direction of the bigger force. 18 N right

Answer: 18 N to the right

Example 2

A parachutist falls at a steady speed. Her weight is 700 N. What is the air resistance?

  1. Steady speed means the forces are balanced.
  2. So air resistance equals weight. 700 N up

Answer: 700 N