You kick a ball over a flat field. Taking into account gravity, but disregarding everything else like wind, friction, bounces, etc., etc., at what angle should you kick it so the ball lands the farthest away from you? And at what angle should you kick it so the ball makes the longest trajectory before landing?
(In reply to
re: Proof that the second part needs a numberical solution by Charlie)
A plot of
dL/da ~ 1  sin a * ln[tan a + sec a] = f(a)
reveals that there are two solutions for f(a) = 0, one at 56.5 and
another one at 56.5. In fact, the plot indicates that f(a) is a
symmetric function. It was not obvious to me at first, but it can be
proved that f(a) = f(a).
That being said, I do not know why the iterative method Charlie uses to
solve the ecuation only converges to the solution at 56.5. The program
is still usefull; since f(a) = f(a) we know that if 56.5 is a
solution, then 56.5 must also be a solution (even though we can't get
to it).
On the other hand, if one uses Newton's iterative formula,
a = a_0  f(a_0)/f'(a_0)
then the convergence is better behaved, although the program becomes more complicated because of the new f'(a_0) term.

Posted by ajosin
on 20050320 04:04:34 