title: Math&JS: sinusoidal animation
body: <p>given incremental values form 0 to infinite
  I'd like to get back a value that increase, then decrease, then increase, then decrease...like a sine</p>
  
  <p>I'm basically trying to get an animation of a ball that goes up and down, based on input values that are only linearly increasing (this value is given by scrolling the page for example to from 0px to TBD +1)</p>
  
  <p>what the math function should I use?</p>
  
  <p>thanks</p>