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Homework Statement
A single large circular Olympic ring hangs freely at the lower end of a strong flexible rope firmly supported at the other end. Two identical small beads each of mass 30 kg are free to slide symmetrically without friction around the ring (the ring passes through the holes in the beads). The two beads are released from rest at the very top as shown and they slide symmetrically down around opposite sides of the ring. At least once during their fall to the bottom of the tension in the rope is observed to be zero. Calculate the maximum value for the mass of the ring. Answer in kg.
a) 15
b) 20
c) 25
d) 30
e) This could never happen
The question setup
Homework Equations
[itex]F_c = \frac {mv^2}{r}[/itex]
The Attempt at a Solution
My final answer was 30 kg (d), but I am unsure if my steps and logic are correct.
The beads slide down a circular ring, so they will exert a centrifugal force to the ring that will counter gravity.
Since centrifugal force acts away from the center, the tension can only be zero when the beads are at the top half of the ring.
The free body diagram of the beads will look like this.
[itex]F_c = \frac {mv^2}{r}[/itex]
X components cancel since the 2 beads are in opposite directions.
Y component: [itex]F_cy = \frac {mv^2}{r} * sin \theta[/itex]
Solve for v2 using the work-energy theorem (setting the zero point at the diameter of the ring).
[itex]mgr = \frac {mv^2}{2} + mgrsin\theta[/itex]
[itex]v^2 = 2gr - 2grsin\theta [/itex]
Sub v2 into the y component of the centrifugal force...
[itex]F_cy = m\frac {2gr - 2grsin\theta}{r} * sin \theta[/itex]
[itex]F_cy = 2mgsin\theta - 2mg sin^2 \theta[/itex]
Differentiate the y component with respect to θ and set to zero.
[itex]0 = 2mgcos\theta -2mg (2sin \theta)(cos\theta)[/itex]
[itex]0 = 1 -2sin \theta[/itex]
[itex]sin\theta = 0.5 [/itex]
Sub sinθ = 0.5 into the y component
[itex]F_cy = 2mgsin\theta - 2mg sin^2 \theta[/itex]
[itex]F_cy = 2(30)g(0.5) - 2(30)g(0.25)[/itex]
[itex]F_cy = 15g[/itex]
There are 2 beads, so the total force upwards is 30g.
30g = Mg (M is the mass of the ring)
M = 30
Again I am not sure if I did the question correctly. I used centrifugal force instead of centripetal force, and in all the physics questions I've done I've never used centrifugal force in a free body diagram. Also I am not sure if the force of gravity from the beads would act on the ring.