Variable Mass And Newton's Second Law

Using the momentum form of Newton's Second Law from NCERT Class 11, Chapter 5 (Laws of Motion), force equals the rate of change of momentum, F = dp/dt, which is especially useful when mass accumulates over time. The incoming rain brings downward momentum at a rate equal to the mass arrival rate multiplied by the impact speed: dp/dt = (dm/dt) × v = 2 × 5 = 10 kg·m/s per second. To bring this downward momentum to zero each second, the ground must supply an equal upward force of 10 N in addition to supporting the static weight already present. Option 2.5 N divides instead of multiplying the rate and speed. Option 20 N doubles the result, perhaps assuming the water rebounds. Option 5 N uses only the speed and forgets the mass rate. A conceptual caution is that this analysis assumes the water sticks to the cart on impact rather than bouncing; if it rebounded elastically, the momentum change and hence the force would be larger. The same momentum-rate reasoning underlies how rockets and conveyor belts are analysed, where mass is steadily ejected or added over time. Plausibility check: the units (kg/s)·(m/s) give kg·m/s², which is newtons, and a modest rain rate producing about 10 N of extra force is physically reasonable, confirming the answer.