The idea of inertia is not new. It was not invented by Newton, or even by Galileo. Galileo did make a fascinating elaboration on it, though. Let's look at the famous thought experiment in which he talked about dropping balls of different sizes from the Tower of Pisa. The point of the experiment is that, although there are in principle two ways of measuring mass, they come out to be the same. One can measure mass by inertia, or one can measure it in terms of gravity. If mass (gravity) is the important variable in our thought experiment, then one would expect the heavier ball -- say a cannon ball -- to fall to the ground more quickly than the small one, say, the pebble. On the other hand, if mass (inertia) in the important variable than one would expect the cannonball to move more slowly than the pebble. Inertial mass holds it back from the free fall the pebble enjoys. The only way that the cannonball and the pebble fall at the same ...
