136 completely trivial point. Masses can be measured against each other without using gravity at all , for example far into space, by comparing their relative accelerations when subject to a standard force, a push. If one object accelerates at half the rate of another when subject to our standard push, we conclude it has twice the mass. Thinking of the mass in this way as a measure of resistance to having velocity changed by an outside force, Newton called it inertia . (Note that this is a bit different from everyday speech, where we think of inertia as being displayed by something that stays at rest. For Newton, steady motion in a straight line is the same as being at rest. That seems perhaps counterintuitive, but that’s because in ordinary life, steady motion in a straight line usually causes some frictional or resistive forces to come into play). 20.13 Mass and Weight To return to the concept of mass, it is really just a measure of the amount of stuff . For a uniform material, such as water, or a uniform solid, the mass is the volume multiplied by the density—
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This note was uploaded on 01/08/2012 for the course PHY 322 taught by Professor Daser during the Spring '09 term at SUNY Stony Brook.