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11 January, 12:12

A lightweight string is wrapped several times around the rim of a small hoop. If the free end of the string is held in place and the hoop is released from rest, the string unwinds and the hoop descends. How does the tension in the string (T) compare to the weight of the hoop

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  1. 11 January, 12:30
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    Let T be the tension

    For linear motion of hoop downwards

    mg - T = ma, m is mass of the hoop. a is linear acceleration of CG of hoop.

    For rotational motion of hoop

    Torque by tension

    T x R, R is radius of hoop.

    Angular acceleration be α,

    Linear acceleration a = α R

    So TR = I α

    = I a / R

    a = TR² / I

    Putting this value in earlier relation

    mg - T = m TR² / I

    mg = T (1 + m R² / I)

    T = mg / (1 + m R² / I)

    mg / (1 + R² / k²)

    Tension is less than mg or weight because denominator of the expression is more than 1.
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