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Gravitational mass and inertial mass are the same only in theBut the equivalence between the gravitational and inertial masses is
gravitational field alone. All forces other than gravity tend to mask this
equivalence. One of the reasons of discrepancy with horological
corrections for a physical pendulum might be the dependence of the forces
involved on shape of moving object (a drag force on the swinging bob
depends on its velocity AND on its size, and even on degree of smoothness
of its surface if we go for high accuracy). An open parachute is a good
illustration. For a class demo I used sometimes a box with chalk and a
sheet of paper dropped together, and in the second trial did the same with
the same sheet thoroughly crumpled so that now both items hit the floor
simultaneously. By changing the shape of the sheet we could uncover the
equivalence of the inertial and gravitational mass. My personal feeling is
that the students were impressed by this simple demo.