# Example and question

1. Sep 11, 2009

### ghost11

Hi guys
is the example true?
what will the clock 3 count when the light reach it

Last edited: Sep 11, 2009
2. Sep 11, 2009

### tiny-tim

Welcome to PF! Move towards the light!

Hi ghost11! Welcome to PF!

Yes, the diagrams are correct …

the lamp and the clocks are stationary, and the plane is going half as fast as the light, so in the clocks' frame (middle diagram), the plane will reach the first clock at the same time as the light reaches the second clock.

And in the plane's frame (bottom diagram), a ray of light is moving left, and the clocks are moving right, half as fast as the light …

(and of course they're closer together, but that won't make any difference)

so when the first clock reaches the plane, the third clock will have moved towards the light.
You do the maths (no square-roots needed! ) …

what equations do you get?

Hint: move towards the light, ghost11!

3. Sep 11, 2009

### ghost11

thank you tiny-tim
in the second and third diagram the clocks count 1 second!!
in the second diagram it will take 1.5 second for the light to reach clock 3
in the third diagram it will take 1 second for the light to reach clock 3
what the clock will count for real??
please explane more ... cause iam little slow ..

4. Sep 11, 2009

### Fredrik

Staff Emeritus
Clock 3 will show 1.5s when the light reaches it. The third diagram is a bit confusing. It shouldn't say "clocks 1s". It should say that the clock on the plane shows 1s. Also, it shouldn't say that the plane's speed is c/2. It should say that the speed of the light bulb and the clocks in the picture is c/2.

5. Sep 11, 2009

### ghost11

Fredrik thank you a lot
I didn't see it that way ... i understand now