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Let Rbe rectangular region with vertices (0,0),(b,0),(0,h),and(b,h)

If the density at each point in R is proportional to the point’s distance from the y-axis, find the moments of inertia about the x- and y-axes. Use these answers to find the radii of gyration of Rabout the x- and y-axes.

Short Answer

Expert verified

The moment of inertia is Iy=14khb4and Ix=16kb2h3.

The mass is m=12khb2

The radius of gyration isRy=b2andRx=h3

Step by step solution

01

Given Information

It is given that vertices of rectangular region is (0,0),(b,0),(0,h)and(b,h)

ÒÏ(x,y)=kx

02

Calculation of Iy

The formula is Iy=∬Ωx2ÒÏ(x,y)dA

Iy=∫0b∫0hx2ÒÏ(x,y)dydx

Iy=∫0b∫0hx2kxdydx[ÒÏ(x,y)=kx]

Iy=k∫0b∫0hx3dydx

Solving inner integral

Iy=k∫0b[y]0hx3dx=k∫0b[h]x3dx=kh∫0bx3dx

Iy=khx440b=khb44

Hence,Iy=14khb4

03

Calculating Ix

The formula is Ix=∬Ωy2ÒÏ(x,y)dA

Imposing limits

Ix=∫0b∫0hy2ÒÏ(x,y)dydx=∫0b∫0hy2kxdydx=k∫0bxy330hdx

Ix=k∫0bxh33dx=13kh3∫0bxdx

Ix=16kb2h3

04

Mass of Lamina

Mass of Lamina is given by m=∬ΩÒÏ(x,y)dA

As ÒÏ(x,y)=kx

⇒m=∫0b∫0hkxdydx

m=∫0bkx[y]0hdx

m=∫0bkxhdx

m=kh∫0bxdx

Solving further

m=khx220b

Mass ism=12khb2

05

Radius of Gyration

Radius of gyration is Ry=IymandRx=Ixm

Ry=14khb412khb2andRx=16kb2h312khb2

⇒Ry=b2andRx=h3

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