# Modulus for X axis.

1. Oct 1, 2008

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. Oct 1, 2008

### HallsofIvy

Staff Emeritus
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 $\lambda$, that, at time 0 (say, when I start the experiment) has mass m= A grams, then as time t, it will have mass $m= A(1/2)^{\lambda t}$. 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.

3. Oct 1, 2008