# Different values for the inertia of a moving object?

I would appreciate terse responses to the following statement:

"Relativity requires different values for the inertia of a moving object: in its direction of motion, and perpendicular to that direction. This contradicts the logical principle that the laws of physics are the same in all directions."

Thanks.

pervect
Staff Emeritus
Relativity says that when you transform into a frame in which you are standing still, space has the same properties in all directions. Which is true, and necessary for isotropy.

A lot of people assume that if you have all the laws of physics, the Newtonian laws apply without any modification. This is incorrect and a source of confusion.

Another way of saying this is that the statement is not a statement about the laws of physics, but about human choices, coordinate choices. If it were about the laws of physics, there would be some sort of experiment that tells you which frame is moving, and which isn't. But the point of relativity is that there isn't.

PAllen
To add to Pervect, the idea of transverse and longitudinal relativistic mass derives from the choice to use F=ma, where 'a' is spatial acceleration. To my mind, this is as silly as trying to come up with compensations to mechanics such that you can use normal velocity addition in SR rather than relativistic velocity addition.

If, instead, you use 4-vectors, you have:

p = m U (U being 4-velocity)

and

F = dp/d tau (F now being 4 force)

with no need for any transverse and longitudinal relativistic mass.

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jtbell
Mentor
"Relativity requires different values for the inertia of a moving object: in its direction of motion, and perpendicular to that direction. This contradicts the logical principle that the laws of physics are the same in all directions."

Are you quoting this from Conservapedia's "Counterexamples to Relativity" page?

Are you quoting this from Conservapedia's "Counterexamples to Relativity" page?

Yes - that's where I found this. Supposedly, CP's founder has a BSEE from Princeton... (scratching head).

PAllen