This is a question concerning a trick I observed while solving for the angle responsible for maximum range of a projectile.
What I have observed is :
If you draw two lines, one opposite to the acceleration faced by the body(i) , another in the direction we want to find the projectile(ii), the angle for maximum range is always half the angle between the two lines .
In other words , the body should be thrown along the angle bisector of the two lines mentioned above to get the maximum range .
We note that this trick works for all the normal projectile cases because we know the angle should be 45° .
For an example , let's say a body is projected and faces acceleration due to gravity , 'g' in both vertical and horizontal directions. Line (i) will be at 135° from the horizontal opposite to the resultant acceleration √2g and line (ii) will be along 0° from the horizontal. So 67.5° is the angle for maximum range which is also the result derived after differentiating the range w.r.t the angle .
For projections along/down inclined planes as well , the trick seems to work. If x° is the angle of the inclined plane,line (i) is along y-axis and line (ii) ,x° from the horizontal. The angle for maximum range is thus (90°-x)/2 = 45°-x/2 from the inclined plane .
For down the incline , line (i) is as usual 90° from the horizontal and rotating line(i) through 90°+x° gives us line(ii) . The angle for maximum range is thus 45°+x/2 from the inclined plane .
I would like to know if there is some solid mathematical proof behind this trick . Why does it seem to work everytime ? Are there any cases where it doesn't?
I am sorry if I am missing something trivial .