A 0.2 kg ball moving at 10 m per second is brought to rest in 0.05 s. The average force on the ball is:

1. Initial momentum p_i = m·u = 0.2 × 10 = 2 kg m per second. 2. Final momentum p_f = 0 (ball at rest). 3. Change in momentum Δp = p_f − p_i = −2 kg m per second. 4. Impulse F·t = Δp, so F = Δp / t. 5. F = (−2) / 0.05 = −40 N (the sign means decelerating force). 6. The magnitude of the average force is 40 N. _Source: Selina Concise Physics ICSE Class 9, Ch 3 'Laws of Motion' (aplustopper.com extract)_