Q.

A very long uniform helix is made of thin metal wire. The axis of helix is vertical. A small bead begins to slide down the fixed helix starting from rest. Considering friction between bead and wire of helix to be nonzero, which of the following statements is/ are true as long as bead moves on helix.

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a

The speed of bead first increases and then remains constant.

b

The magnitude of frictional force increases and then remains constant.

c

The speed of bead keeps on increasing. 

d

The magnitude of frictional force on bead remains constant. 

answer is C, D.

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Detailed Solution

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The component of weight in direction of velocity of bead remains constant. Since speed increases, the the horizontal component of normal reaction keeps on increasing. Hence friction force increases till the bead acquires terminal velocity.

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