Re: Help w/ Relativity Problem?

["Stanley J. McCaslin" <mccaslin@pscosf.peru.edu>]

At 10:51 AM 4/2/96 EST, you wrote:
> > 	A spaceship has a length of 200m in its own reference 
> > 	frame.  It is traveling at 0.95c relative to Earth.  
> > 	Suppose that the tail of the spaceship emits a flash 
> > 	of light.  (a)In the reference frame of the spaceship, 
> > 	how long does the light take to reach the nose?  
> > 	(b)In the reference frame of the Earth, how long does 
> > 	this take?  Calculate the time directly from the motions of 
> > 	the spaceship and the flash of light, and explain 
> > 	why you cannot obtain the answer by applying the 
> > 	time-dilation factor to the result from Part (a).
> > 		(from Ohanian's Principles of Physics)
> >
> > I would rather have the 
> > understanding of the problem and work from there.
                    -6
I also get 4.16 X 10   sec.  Could someone please tell me what I'm missing
in this analysis:

> From the earth's viewpoint, the nose is 200sqrt(1 - .95^2) or 62.45 meters
ahead of the tail when the flash is emitted.  Also from earth's view, the
nose travels 0.95ct during the light's flight time.  So the distance is
        62.45 + 0.95ct

t = distance / c 

t = (62.45 + .95ct)/c
                           -6
t = 62.45/0.05c = 4.16 X 10   sec.

The distance the light travels, from earth viewpoint is 1248 meters.

An observer on the ship would say the light traveled 200 + .95cT
or 200 + .95c(6.67e-7)c = 390 m

1248sqrt(1 - .95*.95) = 390  (it seems consistant to me, making Ohanian's
answer wrong).
Stanley J. McCaslin			Inet: mccaslin@pscosf.peru.edu
Assistant Professor, Computer Science	Phone:402/872-2208
Peru State College, Peru, NE  68421     Home: 402/872-7595