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Jacobpm64
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Homework Statement
A particle is projected vertically upward in a constant gravitational field with an initial speed [tex] v_0[/tex]. Show that if there is a retarding force proportional to the square of the instantaneous speed, the speed of the particle when it returns to the initial position is [tex] \frac{v_0 v_t}{\sqrt{v_0^2 + v_t^2}} [/tex] where [tex] v_t [/tex] is the terminal speed.
Homework Equations
Newton's 2nd law: [tex] F_{net} = m \ddot{z} [/tex]
The Attempt at a Solution
Since our retarding force is proportional to the square of the velocity, [tex] F_{r} = kmv^2 [/tex]
Now putting this and gravity into Newton's 2nd law, we have:
[tex] -mg - kmv^2 = m \ddot{z} [/tex]
therefore, [tex] \ddot{z} = -g-kv^2 [/tex]
Separating variables and noticing that [tex] \ddot{z} = v [/tex], we have:
[tex] \frac{dv}{g+kv^2} = -dt [/tex]
Integrating,
[tex] \frac{tan^{-1}(\frac{\sqrt{k}}{\sqrt{g}}v)}{\sqrt{g}\sqrt{k}} = -t + c_{1} [/tex]
At [tex] t = 0 [/tex], [tex] \dot{z} = v = v_0 [/tex]
Therefore, solving for [tex] c_1 [/tex] and plugging back in, we get:
[tex] \frac{tan^{-1}(\frac{\sqrt{k}}{\sqrt{g}}v)}{\sqrt{g}\sqrt{k}} = -t + \frac{tan^{-1}(\frac{\sqrt{k}}{\sqrt{g}}v_0)}{\sqrt{g}\sqrt{k}} [/tex]
Solving for [tex] v [/tex], I get:
[tex] v = \frac{\sqrt{g} tan({tan^{-1}[\frac{\sqrt{k}}{\sqrt{g}}v_0] - t \sqrt{g} \sqrt{k}) }}{\sqrt{k}} [/tex]
Integrating to find the displacement equation , we get:
[tex] z = \frac{ln(cos(\sqrt{g}\sqrt{k} t - tan^{-1}(\frac{\sqrt{k}}{\sqrt{g}}v_0)))}{k} + c_2 [/tex]
When [tex] t = 0 [/tex], [tex] z = 0 [/tex], so solving for [tex] c_2 [/tex] and plugging back in, we get,
[tex] z = \frac{ln(cos(\sqrt{g}\sqrt{k} t - tan^{-1}(\frac{\sqrt{k}}{\sqrt{g}}v_0)))}{k} - \frac{ln(\frac{g}{g+kv_0^2})}{2k} [/tex]
So, now I set this equal to [tex] 0 [/tex] so that i can find the time when the projectile returns to its initial position of [tex] z = 0 [/tex].
Solving for [tex] t [/tex] when [tex] z = 0 [/tex], I get some extremely nasty stuff according to Mathematica.
The equations start getting so nasty, I can't really do anything with them. Much less take the limit of v as t -> infinity to try to find the terminal velocity.
There has to be an easier way to do this, as my professor never even mentioned using Mathematica or computers in general.
Any help with attacking this problem would be appreciated extremely. (I can give the rest of the silly equations I got at the end if anyone wants, but it got so long that it wasn't helpful at all i don't think).