bullet enters a wall with speed v1 and goes out with speed v2, the wall is h units thick. During that time it experiences an impending force which is proprtional to v^2. I have to find the time the bullet goes out the wall. I can take t0=0 v0=v1 x0=0 Using N2 law i get d(mv)/dt = -F..............mdv/dt = -m*v^2 (im not sure if im missing here something) dv/dt = -v^2 dt = - dv/v^2 ===> t = (v1-v2)/v1*v2...this result is wierd since it doesnt contain h so i must have constructed the equation wrongly i guess. I think that F depends on h since F can act only between [0; h], and maybe the answer should be multiplied with h and t = h*(v1-v2)/v1*v2...but it still doesnt look right to me....any opinions?