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Show that moment of inertia of a disk of mass M and radius R is 12MR2. Divide the disk into narrow rings, each of radius r and width dr. The contribution I of by one of these rings is r2dm, where dm is amount of mass contained in that particular ring. The mass of any ring is the total mass times the fraction of the total area occupied by the area of the ring. The area of this ring is approximately 2Ï€°ù»å°ù. Use integral calculus to add up all the calculations.

Short Answer

Expert verified

It is proved that the moment of inertia of disk is 12MR2.

Step by step solution

01

Identification of given data

The mass of disk is M.

The radius of disk is R.

The radius of each ring is r.

The width of each ring is dr.

The mass of each ring is dm.

The moment of inertia of each ring is I=r2dm.

02

Conceptual Explanation

The moment of inertia of the disk is obtained by calculating mass of each ring then substitute in the formula for mass of each ring.

03

Determination of moment of inertia of disk

The density of disk is given as:

ÒÏ=MÏ€R2r

The volume of each ring is given as:

V=2Ï€rdrrdV=2Ï€r2dr

The mass of each ring is given as:

dm=ÒÏdVdm=MÏ€R2r2Ï€r2drdm=2MR2rdr

The moment of inertia of disk is calculated as:

Id=∫0RIId=∫0Rr2dmId=∫0Rr22MR2rdrId=2MR2∫0Rr3dr

Id=2MR2r440RId=2MR2R44Id=12MR2

Therefore, the moment of inertia of disk is 12MR2.

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