Velocity is the variable which is changing during motion of skater.
Velocity is the variable which is changing when the skater moves up and down the track because moving upward on the slope decrease the speed of the skater while on the other hand, velocity increases when moving downward on the slope.
Mass of the body which is skating remain constant or can't change with height. Mass is the amount of matter present inside the body so we can conclude that velocity is the variable which is changing during motion of skater.
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