You need a control which means lots of other phones. Otherwise you could get false positives.
Imagine this scenario. It's the night of the big game. Everyone stays up to watch it. You plug your phone into the charger and place it on the night stand next to the bed. You then start having sex and the motions cause enough vibration to trigger the app. Now imagine everyone doing the same. Right about the time you are about to finish you get a text saying we're all going to die.