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Question about Inertial Frames... Please Help! (1 Viewer)

meeatu

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Hey.
We have just recently gone through the concepts of time dilation and length contraction in physics.
My problem lies in mass dilation, wherein, our teacher mentioned that mass dilation is observable from within the frame of reference that it applies to (the rest frame) - Unlike mass dilation and length contraction which are only apparent relative to an external frame that has a relative velocity to the rest frame.

My question is: Surely this contradicts one of the key principles of special relativity, as it presents an experiment that you can perform to determine if you are in an inertial frame of reference that is at rest or at moving with constant velocity, without a reference point in another frame.

Could someone please explain this?
I do hate it when physics drops it's pen and walks off...

Another short question (of less importance):
We were given the apparent paradox of the barn and the fence that Einstein came up with, and figured out that the obvious problem in it lay in the idea of simultaneity... But Isn't there another reason to why the fence should not fit into the barn...? Surely from the fences perspective, the barn (which would have relativistic velocity) would appear shorter, and thus, the fence should not fit anyway?

-Have I missed some key concept to special relativity here?-
*panics*
 

study-freak

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Hey.
We have just recently gone through the concepts of time dilation and length contraction in physics.
My problem lies in mass dilation, wherein, our teacher mentioned that mass dilation is observable from within the frame of reference that it applies to (the rest frame) - Unlike mass dilation and length contraction which are only apparent relative to an external frame that has a relative velocity to the rest frame.

My question is: Surely this contradicts one of the key principles of special relativity, as it presents an experiment that you can perform to determine if you are in an inertial frame of reference that is at rest or at moving with constant velocity, without a reference point in another frame.

Could someone please explain this?
I do hate it when physics drops it's pen and walks off...

Another short question (of less importance):
We were given the apparent paradox of the barn and the fence that Einstein came up with, and figured out that the obvious problem in it lay in the idea of simultaneity... But Isn't there another reason to why the fence should not fit into the barn...? Surely from the fences perspective, the barn (which would have relativistic velocity) would appear shorter, and thus, the fence should not fit anyway?

-Have I missed some key concept to special relativity here?-
*panics*
1st Q: Mass dilation and length contraction applies only when an object moving relative to an observer is seen by the observer. Hence, if you look at the object (you are the observer), you can figure out your frame's motion, in which case you already referred to an outside frame of reference (the object). If you don't look at anything, you didn't refer to an outside frame and your motion is unknown (given that you are travelling at a constant velocity).
Hence the principle of relativity is obeyed.

2nd Q: lol, what? barn and fence? never heard of it.
Give us the paradox thing first?
 

meeatu

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1st Q: Mass dilation and length contraction applies only when an object moving relative to an observer is seen by the observer. Hence, if you look at the object (you are the observer), you can figure out your frame's motion, in which case you already referred to an outside frame of reference (the object). If you don't look at anything, you didn't refer to an outside frame and your motion is unknown (given that you are travelling at a constant velocity).
Hence the principle of relativity is obeyed.

2nd Q: lol, what? barn and fence? never heard of it.
Give us the paradox thing first?

I knew all that, but my teacher seemed to be under the impression that mass dilation could be measured from WITHIN the frame of reference that it applies to... i.e: If you go really fast, you can measure your own mass to be increasing, without any reference to external frames. That was what made no sense to me.

As for the fence and the barn thing, the apparent paradox was that a farmer stands outside his barn (with a good knowledge of physics, as farmers all have :p) and wants to store his 50m fence into his 30m barn... He knows that as you approach the speed of light, your length dilates, and so he gets his friend Pete (who happens to be able to run at large fractions of the speed of light) to carry the fence and run at the open barn doors. He does not expect the fence to remain smaller relative to the barn after Pete has stopped, but he wants to see if even for a fraction of a second he can get it to fit.

Long story short is, it can't work due to simultaneity ensuring that they both see different things... But my question is, from the perspective of the fence (I.e: the fence itself is the rest frame, and the barn has relative velocity to it) surely, the barn would appear less long, as, relative to the fence (and to Pete) it has great velocity?
 

Dx_God

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either u missed out on a few words or ur teacher didn't really get this part. u WILL NOT find mass dilation within the reference frame but however u CAN measure the mass dilation as measured by an observer from ANOTHER reference frame. i think that's what ur teacher is trying to tell u.
 

study-freak

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I knew all that, but my teacher seemed to be under the impression that mass dilation could be measured from WITHIN the frame of reference that it applies to... i.e: If you go really fast, you can measure your own mass to be increasing, without any reference to external frames. That was what made no sense to me.

As for the fence and the barn thing, the apparent paradox was that a farmer stands outside his barn (with a good knowledge of physics, as farmers all have :p) and wants to store his 50m fence into his 30m barn... He knows that as you approach the speed of light, your length dilates, and so he gets his friend Pete (who happens to be able to run at large fractions of the speed of light) to carry the fence and run at the open barn doors. He does not expect the fence to remain smaller relative to the barn after Pete has stopped, but he wants to see if even for a fraction of a second he can get it to fit.

Long story short is, it can't work due to simultaneity ensuring that they both see different things... But my question is, from the perspective of the fence (I.e: the fence itself is the rest frame, and the barn has relative velocity to it) surely, the barn would appear less long, as, relative to the fence (and to Pete) it has great velocity?
Yes, that's true.
This relativity principle works in both ways as motion is relative.
 

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