- #1
Mr Davis 97
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Homework Statement
An athlete stands at the peak of a hill that slopes downward uniformly at angle ##\phi##. At what angle ##\theta## from the horizontal should they throw a rock so that it has the greatest range?
Homework Equations
SUVAT equations and trig relations
The Attempt at a Solution
I found the following three equations:
##y = v_0 \sin \theta t - \frac{1}{2} g t^2##
##t = \frac{x}{v_0 \cos \theta}##
##y = -x \tan \phi##
I combined all of these to find the final horizontal position x:
##x = \frac{v_0^2}{g}(\sin 2 \theta + 2 \tan \phi \cos^2 \theta)##
We need to maximize ##x## with respect to ##\theta##, so we take the first derivative and set it to ##0##.
##\frac{dx}{d \theta} = 2 \cos 2 \theta + 2 \tan \phi (2 \cos \theta) (- \sin \theta) = 0##
##\tan 2 \theta = \frac{1}{ \tan \phi}##
So this is what I end up with, but I am not sure what to do next... Any ideas?
Edit: I played with it a little more and found that ##\theta = \frac{1}{2} (90 - \phi)##, but I am not quite sure how to use this to find the angle that gives the maximum distance.