A 1000 kg car is brought to rest in 5 seconds from 20 m per second. The braking force needed is:

1. Compute change in momentum: Δp = m·(v − u) = 1000·(0 − 20) = −20000 kg m per second. 2. Apply impulse-momentum theorem: F·t = Δp. 3. F = Δp / t = (−20000) / 5. 4. F = −4000 N (the minus sign means the force opposes motion). 5. The required magnitude is 4000 N. 6. Options A and B underestimate; D doubles wrongly. _Source: Selina Concise Physics ICSE Class 9, Ch 3 'Laws of Motion' (aplustopper.com extract)_