I have a question about a physics problem, this comes from Barger's Classical Mechanics: A boat is slowed by a drag force, F(v) and its velocity decreases according to the formula [tex]v=c^2(t-t_{1})^2[/tex] where c is a constant and [tex]t_{1}[/tex] is the time at which it stops. Find the force F(v) as a function of v.(adsbygoogle = window.adsbygoogle || []).push({});

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.

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# Boat Drag Force

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