Suppose there is some hollow vertical cylinder from which there passes a thread having two masses connected to its ends.When one mass is rotated in a circular path in the plane at the top of the cylinder then at a particular speed the mass at the bottom become static and doesn't move up or down.from where is that tension to balance the weight comes from? Is there centripetal or centrifugal force on the rotating mass?

Question

anonymous · Answer

Yes the weight of the mass is acting as a centripetal force. think of it as objects rotating in a circle want to shoot off out from the circle and thus require some force to keep them moving in the circle. In this case that force is being provide by the mass's weight which is pulling toward the centre of the circle (really its acting downwards but because the thread is passing through the cylinder its acting in the plane of the rotation. 
At a certain speed v the centripetal force
$$F=mv/r^2  $$
is equal to the force of the eaths gravity acting on the mass
$$F=Mg$$
(here i've used two different M's one for each of the masses.

I hope this helps

anonymous · Answer

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vincent-lyon.fr · Answer

Hi! There's a slight misprint in Leavenotrace's perfect answer: 
$$F=mv^2/r$$

anonymous · Answer

yes there is a centripetal force on the rotating mass . since the mass is rotating in a uniformly accelerated motion there must be a centripetal force which is here provided by the mass of the hanging one. it is similar as a case in which when a car rotates in a horizontal plane friction provides the necessaery centripetal force