Velocity Augmentation Suit (Part 2)

This is a follow-up to Part 1 . The mad scientist enjoys his first flight so much that he hurriedly prepares his suit for a second flight. In the process, he accidentally bumps the dial which controls the α \alpha parameter.

The launch parameters for the second flight are identical to those of the first flight, except for α \alpha . This time, the poor scientist stays in the air for some time before crash landing (at ground level) 1000 meters from the launch point.

To what value did the scientist accidentally adjust α \alpha (to one decimal place)?


The answer is 68.7.

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1 solution

Guilherme Niedu
May 20, 2017

From our previous problem:

v x = 10 e ( α / 150 ) t \large \displaystyle v_x = 10e^{(\alpha/150)t}

v y = ( 1500 / α ) + ( 20 1500 / α ) e ( α / 150 ) t \large \displaystyle v_y = (1500/ \alpha) + (20 - 1500 / \alpha) e^{(\alpha /150)t}

Integrating with respect to time, and considering that at t = 0 t=0 both x x and y y are equal to 0 0 :

x = ( 1500 / α ) e ( α / 150 ) t \color{#20A900} \boxed{\large \displaystyle x = (1500/ \alpha) e^{(\alpha/150)t}}

y = ( 1500 / α ) t + ( 3000 / α 225000 / α 2 ) e ( α / 150 ) t \color{#20A900} \boxed{\large \displaystyle y = (1500/ \alpha)t + (3000/ \alpha - 225000 / \alpha^2) e^{(\alpha /150)t}}

We want x x to be 1000 1000 while y y is 0 0 . So, for x x :

( 1500 / α ) e ( α / 150 ) t = 1000 \large \displaystyle (1500/ \alpha) e^{(\alpha/150)t} = 1000

( i ) e ( α / 150 ) t = 2 α / 3 \color{#20A900} (i) \ \boxed{\large \displaystyle e^{(\alpha/150)t} = 2\alpha /3}

Also, for y y :

( 1500 / α ) t + ( 3000 / α 225000 / α 2 ) e ( α / 150 ) t = 0 \large \displaystyle (1500/ \alpha)t + (3000/ \alpha - 225000 / \alpha^2) e^{(\alpha /150)t} = 0

Plugging (i) on this:

( 225000 / α 2 3000 / α ) ( 2 α / 3 ) = ( 1500 / α ) t \large \displaystyle (225000 / \alpha^2 - 3000/ \alpha) (2 \alpha /3) = (1500 / \alpha) t

( i i ) t = 300 4 α 3 \color{#20A900} (ii) \ \boxed{\large \displaystyle t = \frac{300 - 4\alpha}{3}}

Plugging (ii) in (i):

e ( 300 4 α ) α 450 = 2 α 3 \large \displaystyle e^{\frac{(300-4\alpha) \alpha}{450} } = \frac{2 \alpha}{3}

I've solved this numerically (if there's a different solution, I'm happy to see), and obtained:

α = 68.74 \color{#3D99F6} \boxed{\large \displaystyle \alpha = 68.74}

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