1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

V=? for Relativistic Mass,length contraction & time dilation

  1. Dec 6, 2016 #1
    1. The problem statement, all variables and given/known data
    Velocity Equations for Relativistic Mass,length contraction and time dilation.
    I was able to figure out one. This is not for homework. I want to learn these equations for future reference.

    2. Relevant equations

    3. The attempt at a solution

    Length Contraction : v = c √{1-(l'/lo)^2}
    Time Dilation: ?
    Mass: ?
  2. jcsd
  3. Dec 6, 2016 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Just so I understand how you are thinking so I can answer properly:
    How did you work out the length-contraction equation? Why not do the same thing for time dilation?

    Note: no such thing as "relativistic mass". Used to be a thing but it turns out not to be much of a useful concept.
  4. Dec 6, 2016 #3
    l'/lo = √{1-v^2/c^2}
    (l'/lo)^2 = 1-v^2/c^2
    1-(l'/lo)^2 = v^2/c^2
    √{1-(l'/lo)^2} = v/c
    c√{1-(l'/lo)^2} = v

    Mass increase has not been discontinued in my program.
    Last edited: Dec 6, 2016
  5. Dec 7, 2016 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Why not do the same thing for time dilation?

    OK you need to do relativistic mass for passing exams - just understand that it is an out of date concept.
  6. Dec 7, 2016 #5
    I just copied the equations from wiki to this platform but i now have come to realize they are the same.
    This is pretty self explanatory. i was given equations from my lessons that were essentially the same but all look different so i got confused. I didnt even look at the wiki equations closely until now :sorry:o_O:frown:
    Last edited: Dec 7, 2016
  7. Dec 7, 2016 #6


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member
    2017 Award

    Better still: Point your teachers to my Insight post What is relativistic mass and why it is not used much? or refer them to me. It is simply a concept that is not used in physics today and it is really just confusing people to use it. Unfortunately, the concept permeates much of the introductory physics literature - which often is not written by people with specialist knowledge on relativity - and therefore becomes used by teachers at pre-university and introductory university level.
  8. Dec 7, 2016 #7

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Well done.
    The form of the equations is ##x = \frac{y}{\sqrt{1-z^2}}## and you want to solve for ##z##.
    It does not matter what the actual letters are.

    It is more important to understand how to use the equations - that is the hard part to get your head around.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted