2.In a perfectly inelastic collision, dP/dt only reduces to d(mv)/dt if the collected mass elements have zero initial momentum, which is equivalent to in J. Mallinckrodt, “F does not equal d(mv)/dt,” letter to the editor, Phys. Teach. 48, 360 (Sept. 2010).
2.For example, holds for a raindrop falling through stationary mist (because the mist has zero initial momentum) as in C. E. Mungan, “More about the falling raindrop,” Am. J. Phys. 78, 1421 (Dec. 2010).
2.In contrast, it does not hold for a horizontally moving funnel adding sand into a cart (because the sand is initially moving with the cart's velocity) as in W. Kunkel and R. Harrington, “Modeling the motion of an increasing mass system,” Phys. Teach. 48, 243–245 (April 2010).