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Problems in Mathematical Analysis.pdf - pwp.net.ipl.pt

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Sec 11] Moments. Centres of Gravity Guld<strong>in</strong>'s Theorems 171<br />

and<br />

Whence<br />

Hence,<br />

Solution. We have<br />

f,<br />

' ,. xJ ..<br />

yds-- yV-.v 2 ^=<br />

J y a 2<br />

a<br />

4. Guld<strong>in</strong>'s theorems.<br />

Theorem 1. The area of a surface obta<strong>in</strong>ed by the<br />

plane curve about some axis ly<strong>in</strong>g <strong>in</strong> the. same plane<br />

rotation of an arc of<br />

as the curve and not<br />

<strong>in</strong>tersect<strong>in</strong>g it is equal to the product of the length of the curve circumference of the circle<br />

the curve.<br />

described by the centre of gravity<br />

by the<br />

of the arc of<br />

Theorem 2. The volume of a solid obta<strong>in</strong>ed by rotation of a plane figure<br />

about some axis ly<strong>in</strong>g <strong>in</strong> the plane of the figure and not <strong>in</strong>tersect<strong>in</strong>g it is<br />

equal to the product of the area of this figure by the circumference of the<br />

circle described by the centre of gravity of the figure.<br />

Fig. 59<br />

1727. F<strong>in</strong>d the static moments about the coord<strong>in</strong>ate axes of<br />

a segment of the straight l<strong>in</strong>e<br />

ly<strong>in</strong>g between the axes.<br />

x y<br />

o o

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