# Finding minimum angular speed

## Homework Statement

The figure illustrates Michelson's setup for measuring the speed of light with the mirrors placed on Mt. San Antonio and Mt. Wilson in California, which are 35 km apart. Using a value of 3.00 x 108 m/s for the speed of light, find the minimum angular speed (in rev/s) for the rotating mirror.

delta t = 2d/c

## The Attempt at a Solution

delta t = 2(3500)/3x10^8 = 2.33x10^-4

Now if I'm on the right track, then I'm not sure what my next step should be.. I feel like I should be dividing or multiplying by 8 since I was told the object is an octagon. Am I right??

LowlyPion
Homework Helper
35 km or 3.5 km?

Doesn't the wheel have to advance 1/8 of a revolution by the time it bounces to see it again in the next successive reflecting surface from when it was originated?

Oh, sorry I meant 2(35000)/3x10^8 = 2.333x10^-4

do I then just divide that by 8 to get 2.91x10^-5 ?

LowlyPion
Homework Helper
Oh, sorry I meant 2(35000)/3x10^8 = 2.333x10^-4

do I then just divide that by 8 to get 2.91x10^-5 ?

You found Δt and that is 1/8 of a rev isn't it?

So how long to make a rev?

I took 2.333x10^-4 and multiplied that by 8 to get .0018664 rev/s but that answer is incorrect

LowlyPion
Homework Helper
I took 2.333x10^-4 and multiplied that by 8 to get .0018664 rev/s but that answer is incorrect

The time for a revolution is not rev/s.

Awesome, got it! 535.79. Thanks for the help! :]