The sine curve overlaid on the cosine curve

If you overlay the sine curve over the cosine curve you can see that they are out of sync with each other by 90°.

The overlaid cosine and sine curve is the base graph of measuring sound and vibration in many industries, The Richter scale uses this to measure when a volcano is next to erupt, SAVING LIVES!

NOTE:

This is used in industry to analyse vibrations in all sorts of applications.