Forces, motion and speed
Physics · Key Stage 3 · The School
Speed = distance ÷ time
A car covering 120 m in 10 s has a speed of 12 m/s. Rearrange the same relationship for the other two: distance = speed × time, time = distance ÷ speed. Always check the UNITS agree before calculating — mixing kilometres with seconds is the commonest source of a wildly wrong answer.
Reading a distance–time graph
A straight sloped line means constant speed; the STEEPER the slope, the faster. A flat line means stopped (distance not changing). A curve means the speed is changing. The gradient of a distance–time graph IS the speed — so you can read a speed straight off the paper without a formula.
Balanced and unbalanced
When forces balance, motion does not change: a stationary object stays still, a moving one continues at steady speed. Unbalanced forces cause ACCELERATION — speeding up, slowing down, or changing direction. A car at constant speed has balanced forces (engine thrust equals drag plus friction), which surprises people who expect "moving" to mean "unbalanced".
A distance–time graph rises steeply for 10 s to 200 m, is flat from 10 s to 25 s, then rises gently to 300 m at 45 s. First leg: 200 m ÷ 10 s = 20 m/s. Middle: flat, so the distance is not changing — stationary for 15 s. Last leg: 100 m in 20 s = 5 m/s. The whole story, including the stop, comes from the gradient of each section, and a flat line never means 'stopped moving on the graph' — it means stopped.