A puck is dropped onto a spinning top of radius R with walls at its rim. Assume

Question

A puck is dropped onto a spinning top of radius R with walls at its rim. Assume that all friction coefficients are mu = 0.5, and that the mass of the puck is m. The top spins with angular frequency omega . (See figure to right.) Assuming that the puck does not bounce and makes full contact with the spinning top immediately, what is the direction and magnitude of net force at the moment of initial contact? (Draw a FBD. What forces cancel out and what forces don't?) The puck slides against the spinning top until it hits the wall. While the puck still slides against the wall (i.e. it is not rotating with the spinning top yet), describe the direction of net force on the puck The puck eventually moves with the top (i.e. remains at the same spot on the wall). What is the direction and magnitude of net force on the puck?

Explanation / Answer

a) After the puck makes full contact with the top, weight of the puck and Normal reaction N gets cancelled equal and opposite direction.

    The centrifugal force pushing the puck towards the wall and the friction on the puck act in opposite direction

mw2 r --- away from the center

0.5 mg = towards the center

Net force   = mw2r - 0.5mg

When the puck touches the wall

the normal contact force of the wall  and the centrifugal force away from the center get cancelled.

The puck will have frictional force against the wall surface.

Normal force by the wall = centrifugal force = mw2 r -- r is the distance from the center

As the ball slides against the wall the frictional force = 0.5xmw2r

C) When the ball rotates with the top the direction of the net force is away from the center

               = mw2R

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