# D = vt vs x' = x - vt

billllib
Homework Statement:
(b) Where does this event occur in Alice’s frame of reference?
Relevant Equations:
d = vt
x' = x - vt

Consider Bob in a vehicle moving to the right (positive x direction) at a speed of 100 km/hour with respect to Alice, who is observing Bob go by. Bob passes Alice at time t = 0, and at that instant he observes a flash of light that occurs at a distancexB=25 kilometersahead of him (in his direction of travel). (Remember what “to observe” means: the event is recorded via one of the clocks in a lattice of synchronized clocks in the observer’s frame of reference.

(a) Where does this event occur in Alice’s frame of reference?

Bob continues to travel at 100 km/hour for 30minutes, at which point he observes another flash of light at a distance of 25 km ahead of him.

(b) Where does this event occur in Alice’s frame of reference?

I just have a question about question b.
In order to find b answer I take

current_location_of_Bob = vt = (100)(.5)= 50

current_location_of_second_flash = 25

distance_away_from_Alice= 25 + 50 = 75

Is there a formula to solve this?
what is the different situations when I use d = vt vs Galilean transform?
Also the Galilean transform is only used when I want B to be the stationary observer instead of A in a spacetime diagram?

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Is this Special Relativity? Or classical physics?

Last edited:
billllib
Classical physics.

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I just have a question about question b.
In order to find b answer I take

current_location_of_Bob = vt = (100)(.5)= 50

current_location_of_second_flash = 25

distance_away_from_Alice= 25 + 50 = 75

Is there a formula to solve this?
what is the different situations when I use d = vt vs Galilean transform?
Also the Galilean transform is only used when I want B to be the stationary observer instead of A in a spacetime diagram?

You don't say which frame of reference you are using in each of these calculations. And, you don't say what transformation (if any) you are using.

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Is this Special Relativity? Or classical physics?
I wonder why each observer has a lattice of synchronized clocks if this is a non-relativistic calculation. Does "classical" exclude "non-relativistic" in addition to "quantum"?

billllib
I am using Alice's frame. But I guess the wording that confuses me it is 25 km away from Bob doesn't that make it Bob's frame?

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I am using Alice's frame. But I guess the wording that confuses me it is 25 km away from Bob doesn't that make it Bob's frame?
Well, you can describe any event in any reference frame. Sometimes you get data as measured in one frame and sometimes in another. You have to be careful, therefore, to know what frame you are using and when you have to transform data.

The point is that I didn't see you use the Galilean transformation anywhere.

billllib
But just to confirm even when it says 25km away from Bob it is still in Alice's frame?

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But just to confirm even when it says 25km away from Bob it is still in Alice's frame?
The question tells you that the 25km was measured by Bob.

That said, does it make a difference?

billllib
I think I got it. x' = x- vt 25 - (-100)(.5) = 75 in Alice frame. Thanks for the help.
Sorry for changing this a million times but shouldn't velocity be -(+100 ) = - 100 not -(-100) = + 100?

Last edited:
billllib
I solved the problem.