Objects tend to keep same direction and speed {inertia}| {matter, inertia}. Mass resists motion change. Inertia is resistance to acceleration.
cause
All universe masses exert gravity on object masses. All universe masses contribute to space-time curvature at space-time points. Objects take the geodesic shortest path between two space-time points. Geodesics are paths that keep same direction and speed. Inertia is following geodesics. Space-time curvature depends on mass, so inertia depends on mass. The dependence is the same. Mass and inertia are the same.
examples
It is harder to pull or push more massive object. If someone puts a big rock on your head and hits it with hammer, you do not feel the hit, because big mass changes motion slowly. People can rapidly pull smooth tablecloths out from under dishes on tables, without moving dishes.
Physical Sciences>Physics>Matter>Mass
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Date Modified: 2022.0224