A car decelerates uniformly from 20 m per second to rest in 4 seconds. The retardation is:

1. Retardation is the magnitude of negative acceleration. 2. Use v = u + at with u = 20, v = 0, t = 4. 3. So 0 = 20 + a × 4, giving a = −5 m per second squared. 4. The minus sign means deceleration; the magnitude (retardation) is 5 m per second squared. 5. Hence the retardation is 5 m per second squared. _Source: Selina Concise Physics ICSE Class 9, Ch 2 'Motion in One Dimension' (aplustopper.com extract)_