theory of relativity- inertial frame

Discussion in 'Physics' started by uzair, Jan 29, 2008.

1. uzair Thread Starter Active Member

Dec 26, 2007
110
0
Hi guys!
I am revising the theory of relativity for my test.I am stuck and need help!

The first postulate says:
"The laws of physics are the same in all inertial frames."

I have a problem with the following lines of textbook, if someone can explain them, it would be a favor.

"If the laws of physics were different for different observers in relative motion, the observer could determine from this difference that which of them were stationary in a space and which were moving."

2. beenthere Retired Moderator

Apr 20, 2004
15,808
295
We know that time runs faster with speed, and that length along the longitudinal axis decreases (the Lawrence-Fitzgerald contraction). However, to an observer along for the fast ride, the distance of one meter is unchanged, as is the duration of one second. Those changes are only evident to an external observer in a different frame of reference.

3. uzair Thread Starter Active Member

Dec 26, 2007
110
0
I dont understand what are you trying to say please explain in simple words.

4. Dave Retired Moderator

Nov 17, 2003
6,960
171
A man in a car is travelling at 30 mph. To him inside of the car he is static, i.e he is travelling at 0 mph with respect to the car, however to an external observer he is travelling at 30 mph. Therefore they are two different observers in relative motion with respect to one another.

Dave

5. Papabravo Expert

Feb 24, 2006
12,280
2,723
So that's where the limerick about the "young fellow named Fisk" comes from.

Code ( (Unknown Language)):
1.
2. There was a young fellow named Fisk
3. Whose fencing was exceedinly brisk
4. So rapid his action
5. The Fitzgerald Contraction
6. Soon shortened his rapier to a disk!
7.
One of the few clean ones I know.

6. beenthere Retired Moderator

Apr 20, 2004
15,808
295
Got me by one!

7. thingmaker3 Retired Moderator

May 16, 2005
5,073
8
The single most understandable treatise on the subject I have ever read is called "Relativity, The Special And General Theory" by Albert Einstein. His explanation is eloquent and clear. The first half of the book is understandable by most lay-persons.

8. studiot AAC Fanatic!

Nov 9, 2007
5,003
523
This of course is the crux of the question

Which theory of relativity are you studying?

Einstein had 2, special and general.

I think your book is referring to Special Relativity, which came first and has lots of experimental evidence to support it. It is entirely mechanical in approach. Hence the reference to inertial systems
This replaced Newtonian/ Lagrangian Relativity (yes there was one).

General Relativity is not so fortunate as it attempts to include and account for gravity but doesn't wholely succeed.

What exactly did you not understand about the original quote?

9. integrityco New Member

Feb 1, 2008
3
0
Eistien contiuned to follw the light beam for years to understand this theory. I think you best understand it as he did by observation.. When driving a vehicle notice the dividing lines on the the highway. the appear to be very short only about two feet yet they are actually about 15 feet. Dave example is right on target I have just taken it another step....

DRC

10. uzair Thread Starter Active Member

Dec 26, 2007
110
0
Thanks a lot guys for all those replies (esp thingmaker3 for his link for lay-persons).My test went good.(Because there was no quesrion about relativity in it hehe)

11. Mark44 Well-Known Member

Nov 26, 2007
626
1
This made me think of an experience I had one wintry night in Montana. I was driving north on a road with almost no other traffic when a blizzard started dropping lots of snow. The wind was directly from the north, so I was driving into the teeth of the blizzard. Within just a few miles I was unable to see the centerline on the road or even the side of the road, so I kept slowing down. Pretty quickly I came to a complete stop, but with the snow blowing directly at my windshield, it still seemed that I was clipping right along.

If I hadn't had a speedometer to tell me my speed along the road I wouldn't have been able to tell whether the car and I were moving and the snow was static, or that the car was stopped and the snow was moving.

I think this is what the quoted sentence is trying to get across.
Mark

12. Ryno3030 New Member

Dec 1, 2007
9
0
These statement is what you are having trouble with right? This means that motion is completely relative to an observer. There is no way to say that you are stationary, because an observer in another reference frame can see you moving. If the laws of physics were not the same for all reference frames, a process of deduction could determine which reference frame is completely stationary in a general (non-observer specific) space. Observers in every conceivable motion relative to the stationary position could agree on a stationary point, which is not possible.

Say you were standing along the side of the road, and a friend (call him Bob) was driving a car on the road. From your point of view, the road is stationary and the car is moving. From Bob's point of view, the car is stationary and the road is moving. The only way for the two of you to agree on a stationary object would be to have different laws of physics in each frame.

Hope this helps!