This question already has an answer here:
I often have this thought when stretching my legs on a long haul flight: jump up and down in the cabin and see whether the pilots complain. Alone, it is unlikely my weight would have any noticeable impact on the plane's attitude, and I'd likely annoying some flight attendant or passenger around.
But let's say I manage to get all passengers on the flight to cooperate. Everybody jumps up pretty much the same instance (within reasonable human reaction time). Will the pilots notice its effect?