Balanced and unbalanced forces

1Adding forces

Forces in the same direction add up. A 30 N push and a 20 N pull, both to the right, give a resultant force of 50 N to the right.

resultant force

2Opposite forces

Forces in opposite directions subtract. The resultant is the difference between them, and it acts in the direction of the bigger force.

3Balanced forces

A book's weight pulls it down. The table squashes very slightly and pushes back up just as hard. The forces are balanced: the resultant is zero.

balanced

4Balanced, but moving

Balanced forces don't mean stopped. When the driving force equals the resistance, a car keeps going at a steady speed in a straight line.

5Unbalanced forces

When the forces are unbalanced, the motion changes. A bigger forward force speeds the car up; a bigger backward force slows it down.

unbalanced

6Changing direction

A force at right angles to the motion changes its direction, not its speed. The wire pulls the hammer ball towards the centre; let go, and it flies off in a straight line.

7The skydiver

Air resistance grows as a skydiver speeds up, until it balances her weight and her speed stays steady. Opening the parachute makes it much bigger, so she slows down.