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So I figured [tex]F=ma=mdv/dt=md/dt(c^2(t-t_1)^2)[/tex]. After differentiation, I get [tex]F=2mc\sqrt{v}[/tex]. The solution in the back has a negative sign, and I'm wondering what justifies the negative. I'm assuming it has somethign to do with the fact that it is a drag force and the velocity is decreasing, but it's not entirely clear. Thanks in advance.