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91Ó°ÊÓ

Let the solid in Problem 7 have density =cosθ.

Show that then Iz=310Ma2sin2α.

Short Answer

Expert verified

The required value of IzMis310a2sin2α.

Step by step solution

01

Given information

The density of the solid iscosθ.

02

Concept of spherical coordinates

The spherical coordinates(r,θ,ϕ)is related to Cartesian coordinates(x,y,z) by:

r=x2+y2+z2θ=tan-1(yx)ϕ=cos-1(zr)

The cylindrical coordinates(r,θ,ϕ) is related to Cartesian coordinates(x,y,z) by:

r=x2+y2θ=tan-1(yx)z=z

03

Evaluate the mass of the solid

Calculate the mass of the solid as follows:

M=∫ÒÏdV=∫0ar2dr∫0acosθdθ∫02ττdÏ•=2ττ3a3∫0asinθdθcosθ=ττa33sinα

The moment of inertia is as follows:

Iz=∫ÒÏx2+y2dV=ÒÏr2sin2θdV=∫02ττdϕ∫0αcosθsin3θdθ∫0ar4dr=-2ττa55∫0αsin3θdθsinθ

04

Use the above values to prove .

Write integral as follows:

IZ=-ττa510sin4θ0α=ττa510sin4α

Substitute ττa510sin4αfor Izand ττa53sinαfor M in IzM.

IzM=310a2sin2α

Therefore, the required value of Iz is 310Ma2sin2α.

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