A ball of mass 1 kg moving at 10 m s⁻¹ hits a floor and rebounds with the same speed after an elastic collision. What is the magnitude of the change in its linear momentum?

1. Take the downward direction as positive: initial momentum = 1 x 10 = 10 kg m s⁻¹. 2. After rebound the velocity reverses, so final momentum = 1 x (−10) = −10 kg m s⁻¹. 3. Change Δp = final − initial = −10 − 10 = −20, magnitude 20 kg m s⁻¹. 4. Rule out 0: speed is unchanged but direction reverses, so momentum does change. 5. Rule out 10: that ignores the reversal and uses only one velocity. _Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 1 "Laws of Motion", p.21_