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

Modulus for X axis.

  1. Oct 1, 2008 #1

    madmike159

    User Avatar
    Gold Member

    y = |x| can't go below the y axis because a Modulus is always positive, but can you get a modulus that stops x going negitive? Could this be used for things like radioactive decay where the graph should go in -x but doesn't because you can't have - time?
     
  2. jcsd
  3. Oct 1, 2008 #2

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    x, as a function of y, x= |y| does that. I am not clear why you say "you can't have negative time". There is no such thing as an "absolute" time. In any application of mathematics, to, say, physics, you are free to choose which moment you will call "t= 0". Negative values of t simply mean times before your chosen starting point.

    For example, if I have a radioactive substance, with half-life [itex]\lambda[/itex], that, at time 0 (say, when I start the experiment) has mass m= A grams, then as time t, it will have mass [itex]m= A(1/2)^{\lambda t}[/itex]. Taking t< 0 will give a mass greater than A, which is a perfectly reasonable answer: before time t= 0, it had greater mass than at time t= 0.
     
  4. Oct 1, 2008 #3

    statdad

    User Avatar
    Homework Helper

    "Taking t< 0 will give a mass greater than A, which is a perfectly reasonable answer: before time t= 0, it had greater mass than at time t= 0."

    Unless it was created at some time as a by-product of a nuclear reaction.
     
  5. Oct 2, 2008 #4

    madmike159

    User Avatar
    Gold Member

    Really all I wanted to know is if there is an opposite of the modulus function.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?