ZIMSEC O Level · Physics · Form 3
Speed, velocity and acceleration
- Speed = distance ÷ time. Velocity is speed in a given direction.
- Acceleration = change in velocity ÷ time: a = (v − u) ÷ t, in m/s².
- Deceleration is negative acceleration.
Graphs
- Distance–time: gradient = speed; a horizontal line means stationary.
- Speed–time: gradient = acceleration; area under the graph = distance travelled.
Worked example: A kombi accelerates uniformly from rest to 20 m/s in 10 s, then travels at 20 m/s for 30 s. Find the acceleration and the total distance.
a = (20 − 0) ÷ 10 = 2 m/s²
Distance = triangle + rectangle = ½ × 10 × 20 + 30 × 20 = 100 + 600 = 700 m
Free fall
Near the Earth all objects fall with acceleration g ≈ 10 m/s² if air resistance is ignored. With air resistance, a falling object reaches terminal velocity when air resistance equals weight.
Exam tip: Always check the units on graph axes – minutes and km/h must be converted before calculating.
Check yourself
- A car's speed changes from 10 m/s to 4 m/s in 3 s. Find its acceleration.
- What does the area under a speed–time graph represent?
- Why does a skydiver reach a terminal velocity?
Show answers
- (4 − 10) ÷ 3 = −2 m/s² (a deceleration of 2 m/s²)
- The distance travelled.
- Air resistance increases with speed until it equals weight, so the resultant force (and acceleration) is zero.
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