This is just something I was thinking about the other day: Lets say that we have an object with the mass of 1kg and at t=0: x=0m and v=0m/s. Now we apply a force over time t with the force: F=sin(t)N. that means that a=sin(t). To get v we integrate and get: v = 1-cos(t). Then we integrate for x and get x = t - sin(t). That means that after a few seconds the mass will be way on the x+ side. But why should this be if on average the amount of force in that direction is 0?!