Math Is Fun Forum

Identity

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Registered: 2007-04-18

Posts: 934

circular motion... with a twist!

A puck of mass m = 1.50kg slides in a circle around the center of a frictionless table of radius r = 20.0cm while attached to a hanging cylinder of mass M = 2.50kg by a cord through a hole in the center of the table. What speed keeps the cylinder at rest?

Thanks so much!

JaneFairfax

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Registered: 2007-02-23

Posts: 6,868

Re: circular motion... with a twist!

Identity

Member

Registered: 2007-04-18

Posts: 934

Re: circular motion... with a twist!

Thankyou Jane, that gives the correct answer, but...

I don't understand...

The centripetal force acts towards the centre of the table, and the gravitational force acts down from the centre,

i.e. they both act in the same net direction, so shouldn't the equation be this (wrong equation obviously because of square root)

luca-deltodesco

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Registered: 2006-05-05

Posts: 1,470

Re: circular motion... with a twist!

no.

forget about the puck, the cylinder is hanging down, with centripetal acceleartion acting up the string to the table-top counteracting the cylinder's weight which is acting down , so for the cylinder to be in equilbrium they must be equal. hence, janefairfax's equation

Last edited by luca-deltodesco (2008-04-03 04:43:50)

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Identity

Member

Registered: 2007-04-18

Posts: 934

Re: circular motion... with a twist!

hmmm centripetal acceleration acting up? I guess it makes sense in practice

Dragonshade

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Registered: 2008-01-16

Posts: 147

Re: circular motion... with a twist!

The only force which is considered valid is the force that exerted by an object which is directly connected with the other.

The cylinder exerted Mg on the cord, and the cord exerted Mg on the puck, so the direction of the force should be parellel with the direction of force on the cord which is pointed straight toward the center.

Last edited by Dragonshade (2008-04-03 14:29:10)