[SOLVED] Mechanics question - help please? Hey, I've just signed up here, and not entirely sure if I'm posting in the right place. But I have a Mechanics question, which has confused me, so here goes: 1. The problem statement, all variables and given/known data A bead of mass m is threaded onto a horizontal wire. When moving with speed u, the retarding force of air resistance is known to be ku^2 where k is a constant. Write down an equation of motion for the bead that describes the speed u in terms of the time t, and hence show that: (m/u) = (m/u0) + kt if u = u0 when t = 0. 2. Relevant equations I guess F = ma ? 3. The attempt at a solution Since the retarding force is given by ku^2 I tried letting a = (F/m) = (-ku^2)/m and then writing that u = u0 - (ktu^2)/m, because the speed should be the initial speed plus (acceleration x time) right? But when I re-arrange this equation to get something in terms of (m/u) it doesn't give me the right answer. Am I even on the right lines?