A metaphoric experiment if you will that describes this mechanism.

Suppose we had a kiddie train track 100 m long. We place a student school desk that has track wheels attached to it on the track. We attach a digital timer to the desk and of course a drive mechanism that can maintain a constant velocity. On the track we place two switches that are some distance apart. The first turns the digital timer on and the second turns it off. We put a student in the desk and tell him how far apart the switches are and the total mass of his system. We ask the student to calculate his momentum at the end of the run. At the end of the run he gives us a value for his momentum. We move the desk back to the starting point in order to do another run. Unbeknownst to the student I can remotely control the timer. I reduce the rate of the timer by 10% and start the run again. At the end of the run we ask the student which run had the greatest momentum and of course he says the second. Isn't this situation somewhat like a rock sitting on the surface of the earth. The atoms of the rock are vibrating around and colliding with other atoms fields and the atoms clocks are speeding up and slowing down ever so slightly so that the collective momentum of the atoms points to the center of the earth?

Now look at one mole of any element sitting at the surface of the Earth. Now imagine the atoms of the element traveling back-and-forth on trillions of our little kiddie train tracks, Their tiny clocks speeding up and slowing down as they move up and down in the gravity well. I do not possess the mathematical skills to test this hypothesis but using Avogadro's number it should be possible to calculate the weight of one mole which should be equal to the mass of one mole. As for inertia, if you take the same one mole far out into space where there is very little gravity and give it a one gee acceleration the atoms clocks do just as they did on Earth with the momentum being opposite the direction of acceleration, hence inertia.