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    Some sand is sprinkled onto the cone. The sand oscillates vertically with the frequency of the cone. Keeping the frequency constant, the current is increased. This increases the amplitude of oscillation of the cone.

    At a particular amplitude of oscillation the sand begins to lose contact with the cone.

    By considering the forces acting on a grain of sand, explain why this happens.

    From mark scheme it says "Weight - contact force = m(w^2)x So as x increases, contact force decreases, sand loses contact with cone when contact force = 0"

    But I don't understand why it is m(w^2)x, not -m(w^2)x? as I thought they take upward as negative and losing contact only happens at maximum upward displacement position.
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