How Do You Calculate the Moment of Inertia of a Spinning Disc with Added Mass?

[ems1312]

Homework Statement

A) a horizontal disc of diameter 12.0cm is spinning freely about a vertical axis through its centre at an angular speed of 72 revolutions per minute. a piece of putty of mass 5.0g drops onto and sticks to the disc at a distance of 4.0cm from the centre. The angular speed reduces to 60 revolutions per minute. calculate the moment of inertia of the disc. No external torques are applied to the system during this process.

B) A constant tangential force is now applied to the rim of the disc which brings the disc to rest in 6.0 s. Calculate the magnitude of this force.

C) calculate the rotational energy of the system before and after the putty is added to the disc. comment on your answer.
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Homework Equations

The Attempt at a Solution

HELP! GOT TILL SATURDAY TO UNDERSTAND IT!

 

[rock.freak667]

No attempt? No thoughts?

Is anything conserved when the putty sticks to the disk?

 

[ems1312]

momentum?

 

[rock.freak667]

momentum?

specifically angular momentum. Can you apply the law of conservation of momentum to find the moment of inertia of the disk?