You are riding a broomstick and see a golden ball hovering motionless 30 meters directly in front
of you. The ball suddenly begins dropping in free-fall with no other forces acting on it. You can
fly at a speed of 25 m/s. At what angle must you fly to catch the golden ball?
Assume you reach your flying speed instantaneously and that this question doesn't violate any
copyright laws. Also, it might be helpful to know that 2sin(θ)* cos(θ) = sin(2θ).

Respuesta :

You should fly at an angle of [tex]14.0^{\circ}[/tex]

Explanation:

The motion of the ball is a free fall motion, so the vertical displacement of the ball (in negative directions) is given by

[tex]y=\frac{1}{2}gt^2[/tex]

where

[tex]g=9.8 m/s^2[/tex] is the acceleration due to gravity

t is the time

The horizontal distance you have to cover to catch the ball is

[tex]x = 30 m=d[/tex]

The vertical and horizontal displacement form a right triangle, so that we can write

[tex]tan \theta = \frac{y}{x}[/tex]

where [tex]\theta[/tex] is the direction of your motion. Substituting,

[tex]tan \theta = \frac{gt^2}{2d}[/tex] (1)

We also know that the horizontal position of the person at time t is

[tex]x(t) = v cos \theta t[/tex]

And so, for the time t at which the person catches the ball,

[tex]d=v cos \theta t\\t=\frac{d}{v cos \theta}\\t^2=\frac{d^2}{v^2 cos^2 \theta}[/tex]

Substituting this expression into (1),

[tex]tan \theta = \frac{g}{2d} \frac{d^2}{v^2 cos^2 \theta}\\2 tan \theta cos^2 \theta = \frac{gd}{2v^2}[/tex]

And using

[tex]tan \theta = \frac{sin \theta}{cos \theta}, 2sin \theta cos \theta = sin(2\theta)[/tex]

we find:

[tex]sin (2\theta) = \frac{gd}{v^2}=\frac{(9.8)(30)}{25^2}=0.470\\2\theta = sin^{-1}(0.470)=28.0^{\circ}\\\theta=\frac{28}{2}=14.0^{\circ}[/tex]

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