It's the crush force vibration of dense matter (crucial) minus the wavelength of friction volume against non sucking atmosphereBut what is the force that is pushing it down in the first place? If I slide the rock from the ground to the surface of the water I have not given it any potential energy, and (crucially) energy can't be created from nothing so it should stay on top of the water. That is unless there is some other force at play here?