Light Intensity rate

Math Questttttttt

An object is moving away from the light source at a rate of 4 units/s. Find the rate of change of the intensity of illumination when it is 2 units from the light source.

yeah, this is my answer:
rate of change = k(1/unit²xsecond)
:confused:

Quote:

Originally Posted by mathletes

An object is moving away from the light source at a rate of 4 units/s. Find the rate of change of the intensity of illumination when it is 2 units from the light source.

yeah, this is my answer:
rate of change = k(1/unit²xsecond)
:confused:

The intesity of illumination as a function of distance $r$ from a point source is:

$I(r)=I(1)/r^2$ ,

where $I(1)$ is the intensity of illumination at a distance of $1$ unit from the source.

Now:

$\frac{d}{dt} I(r)=I(1)\frac{-2}{r^3}\ \frac{dr}{dt}$ .

So now you only need to plug in $r=2$ and $\frac{dr}{dt}=4$ to get the answer, in units of
(whatever you are using as units of intensity)/s.

RonL

thank you, i obviously didn't know what i was doing.