# How fast is the beam of light moving along the shoreline when it is 1 km from P?A lighthouse is located on a small island 2 km away from the nearest point P on a straight shoreline and its...

How fast is the beam of light moving along the shoreline when it is 1 km from *P*?

A lighthouse is located on a small island 2 km away from the nearest point *P* on a straight shoreline and its light makes 5 revolutions per minute. (Round your answer to one decimal place.)

____________________km/min.

*print*Print*list*Cite

### 1 Answer

You should sketch a triangle MPN, where MP represents the horizontal distance of 2 km between lighthouse and the point P and NP represents the beam of light.

Evaluating the tangent of angle `hat(NPM)` yields:

`tan hat(NPM) = (NM)/(MP) => tan hat(NPM) = (NM)/2`

You may use the following notation for the angle `hat(NPM)` such that:

`hat(NPM) = theta`

The problem provides the information that `(d theta)/(dt) = 5` rev/min

Differentiating `tantheta = (NM)/2` with respect to time t yields:

`sec^2 theta ((d theta))/(dt) = (1/2) (d(NM))/dt`

You shold evaluate theta for NM = 1 Km such that:

`tan theta = 1/2 => theta = arctan(1/2)`

`sec^2(arctan(1/2))*5 =(1/2) (d(NM))/dt`

You need to evaluate how fast the beam of light moving along the shoreline when it is 1 km from P such that:

`(d(NM))/dt = 10sec^2(arctan(1/2))`

**Hence, evaluating how fast the beam of light moving along the shoreline when it is 1 km from P yields `(d(NM))/dt = 10sec^2(arctan(1/2)).` **