Two beads of mass
each are positioned at the top of a frictionless hoop of mass
with radius
, which stands vertically on the ground. The beads are given a slight push, and they slide down the hoop, one to the right and the other one to the left, as shown . What is the smallest value of
for which the hoop will rise up off the ground at some time during the motion?
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From symmetry we can say that both the beads at any instant will make equal angle the vertical.
Draw the FBD of any bead when it makes angle θ with the vertical. In the diagram assume that ring provides normal force ( N ) towards the center to the bead(and not away from the center).
Taking radial acceleration of the bead to be 0 we can write that m g cos θ + N = R m v 2 By conservation of energy we can write that:- 2 1 m v 2 = m g R ( 1 − cos θ ) v = 2 g R ( 1 − cos θ ) From the previous eqaution we can write that:- N + m g cos θ = R m . 2 g R ( 1 − cos θ ) N = 2 m g − 3 m g cos θ This is the same force which the bead is providing to the ring in the direction making angle θ with the ring.
Let total force provided by the beads is F n e t . F n e t = 2 N cos θ F n e t = 2 ( 2 m g − 3 m g cos θ ) cos θ F n e t = 2 m g ( 2 − 3 cos θ ) cos θ For Maximum F n e t :- ( ( 2 − 3 cos θ ) cos θ ) ′ = 0 cos θ = 3 1 Hence:- F n e t = 3 2 m g This force should be equal to M g to lift the ring:- 3 2 m g = M g M m = 2 3