Re: Physics - relativity problem


"Bluethunder" <zdotcom@xxxxxxxxxxx> wrote in message
news:P4-dnVUYf_T0JbDeRVn-sg@xxxxxxxxxxxxxx
|I am helping my daughter with her HS physics. Last time I touched
physics
| was over 35 years ago in college. So, you will have to excuse my
rustiness.
| But I have been reading and studying the text book So, I can manage
the
| refresher course. Here is the problem that I am having trouble with.
|
| A spaceship A is travelling toward Earth from the right at a speed of
0.8c
| while another ship B is also travelling toward Earth at 0.67c from the
left.
| How fast does ship A approach ship B according to the crew of ship A?
|
| If I apply the velocity addition formula, u = u' + v / ( 1 +
u'v/c^2 ), I
| get u = 0.95c. This seems to make sense since they are approaching
each
| other. But in the text book of this section, it says if the objects
move
| along a straight line, then the velocity could be either positive or
| negative depending on the direction of travel. If I apply the
velocity of
| the ships based on the direction they are travelling, the answer
clearly is
| going to come out different. Which I got u = -0.28c. This seems odd
to me.
| At what point do I use the positive/negative velocity values when
applying
| the formula?
|
| My daughter also mentioned that in the classroom, the teacher roughly
| explained that the inertial reference frames must be considered in
such a
| way that the Earch/ship B reference frame must be considered as the
Earch is
| moving toward ship B. This is where I am falling short. I sure wish
I
| remember this stuff better. Can you help?
|
| Ken
|

The best I can do is advise you to study mathematics and your daughter
to be aware of Einstein's definition of time, which I'm sure her teacher
is not.

[quote]
we establish by definition that the "time" required by a turtle to
travel
from A to B equals the "time" it requires to travel from B to A.
[end quote]
Reference: http://www.fourmilab.ch/etexts/einstein/specrel/www/

Einstein can prove nothing can go faster than a turtle.

Oops!... Did I say 'a turtle'? Sorry...'light'.
No matter, it IS the same math. Just substitute "a turtle" for "light"
and you have it licked.
Now she can have hours of endless fun arguing with her teacher
and getting thrown out of school, but that's ok, she wasn't
learning anything worthwhile anyway. Scary, huh?
Androcles.







.

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