Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Different values for the inertia of a moving object?

  1. Nov 14, 2011 #1
    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."

  2. jcsd
  3. Nov 14, 2011 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    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.
  4. Nov 14, 2011 #3


    User Avatar
    Science Advisor
    Gold Member

    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)


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

    with no need for any transverse and longitudinal relativistic mass.
    Last edited: Nov 14, 2011
  5. Nov 14, 2011 #4


    User Avatar

    Staff: Mentor

    Are you quoting this from Conservapedia's "Counterexamples to Relativity" page?
  6. Nov 14, 2011 #5
    Yes - that's where I found this. Supposedly, CP's founder has a BSEE from Princeton... (scratching head).
  7. Nov 14, 2011 #6


    User Avatar
    Science Advisor
    Gold Member

    Well the idea of transverse and longitudinal relativistic mass is not clearly wrong. In fact, it is fairly common in old books. I would describe the evolution away from it as: if some definitions force such complexity, and other definitions that are at least as simple do not, let's avoid the definitions with adverse side effects.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook