1. The problem statement, all variables and given/known data A flatcar of mass ##m_0## starts moving to the right due to a constant horizontal force F. Sand spills on the flatcar from a stationary hopper. The velocity of loading is constant and equal to ##\mu## kg/s. Find the time dependence of the velocity and acceleration of flatcar in the process of loading. The friction is negligibly small. (Ans: ##v=Ft/(m_0(1+\mu t/m_0))##, ##a=F/(m_0(1+\mu t/m_0)^2)## ) 2. Relevant equations 3. The attempt at a solution The given question can be easily solved by the impulse momentum equation. The reason I posted the question here was to clarify a doubt about directly applying F=ma. For the question in the thread "Sand flowing out of the car", it was suggested that it can be solved by using F=M(t)a(t) but when I try to apply it to this problem, it gives me a wrong answer. Also, I think I was a bit lucky while applying the Impulse-Momentum equation to the problem. If the falling sand had some velocity in the horizontal direction, the answer would have been different. So how do I know when to apply Impulse-Momentum equation or F=Ma? Any help is appreciated. Thanks!