Hello: An empty freight car of mass M starts from rest under an applied force F. At the same time, sand begins to run into the car at steady rate b from a hopper at rest along the track. Find speed when mass of sand m has been transferred. I have attached a drawing. So would this be: Let M(t) = mass as a function of time. So we have M(t) = M + bt And we have F = M(t)(dv/dt) So F(dt) = M(t)dv or dv = F(dt)/(M + bt) So wouldn't we have v = F/b*ln(M + bt)? But somehow this is wrong? Thanks.