Example 1 · level 1
An auto-rickshaw starts from rest and reaches 28 m/s in 7 s. Find its acceleration in m/s².
Show solution
- u = 0 m/s, v = 28 m/s, t = 7 s.
- a = (28 − 0) ÷ 7 = 4 m/s².
Answer: 4
CLASS 9 SCIENCE · MOTION
Acceleration and retardation
Acceleration a = (v − u) ÷ t, in m/s². A negative value means the body is slowing down (retardation).
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Worked examples
Example 2 · level 2
A bus speeds up from 5 m/s to 11 m/s in 6 s on a highway near Ranchi. Find its acceleration in m/s².
Show solution
- u = 5 m/s, v = 11 m/s, t = 6 s.
- a = (11 − 5) ÷ 6 = 1 m/s².
Answer: 1
Example 3 · level 3
A scooter moving at 72 km/h is braked and stops in 4 s. Find the magnitude of its retardation in m/s² (correct to 2 decimal places if needed).
Show solution
- u = 20 m/s, v = 0, t = 4 s.
- a = (0 − 20) ÷ 4 = −5 m/s².
- Retardation (magnitude) = 5 m/s².
Answer: 5