Question

A lighthouse is located on an island 33 miles from the closest point on a straight shoreline. If the lighthouse light rotates clockwise at a constant rate of 99 revolutions per minute, how fast does the beam of light move towards the point on the shore closest to the island when it is 5252 miles from that point

Answer 1

Question:

A lighthouse is located on an island 3 miles from the closest point on a straight shoreline. If the lighthouse light rotates clockwise at a constant rate of 9 revolutions per minute, how fast does the beam of light move towards the point on the shore closest to the island when it is 52 miles from that point

Answer:

The beam of light moves at
`16278\pi` miles/min

Explanation:

This question is illustrated with the attached image

Taking the instructions in the question, one at a time.

A revolution of 9 per minute implies that:

`(d\theta)/(dt) = (9 * 2\pi\ rad)/(1\ min)`

`(d\theta)/(dt) = 18\pi (rad)/(min)`

Take tan of the angle in the attachment:

`tan(\theta) =(52)/(3)`

Differentiate both sides with respect to time

`(d\ tan(\theta))/(dt) =(52)/(3) * (dx)/(dt)`

Rewrite as:

`(d\ tan(\theta))/(d\theta) * (d\theta)/(dt) =(52)/(3) * (dx)/(dt)`

In calculus:

`sec^2(\theta) =(d\ tan(\theta))/(d\theta)` -- Chain rule

So:

`sec^2(\theta) *(d\theta)/(dt) =(52)/(3) * (dx)/(dt)`

In trigonometry:

`sec^2(\theta) = tan^2(\theta) + 1`

So:

`(tan^2(\theta) + 1)(d\theta)/(dt) =(52)/(3) * (dx)/(dt)`

Recall that:

`tan(\theta) =(x)/(3)`

`(((52)/(3))^2 + 1)(d\theta)/(dt) =(1)/(3) * (dx)/(dt)`

`((52^2)/(9) + 1)(d\theta)/(dt) =(1)/(3) * (dx)/(dt)`

`((2704)/(9) + 1)(d\theta)/(dt) =(1)/(3) * (dx)/(dt)`

`((2704+9)/(9))(d\theta)/(dt) =(1)/(3) * (dx)/(dt)`

`((2713)/(9))(d\theta)/(dt) =(1)/(3) * (dx)/(dt)`

Recall that:
`(d\theta)/(dt) = 18\pi (rad)/(min)`

`((2713)/(9)) * 18\pi =(1)/(3) * (dx)/(dt)`

`2713 * 2\pi =(1)/(3) * (dx)/(dt)`

Multiply both sides by 3

`3 * 2713 * 2\pi =(1)/(3) * (dx)/(dt) * 3`

`3 * 2713 * 2\pi =(dx)/(dt)`

`16278\pi =(dx)/(dt)`

`(dx)/(dt) = 16278\pi` miles/min

Hence:

The beam of light moves at
`16278\pi` miles/min