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Two portions of a thin ring each of mass M M and and radii R R are joined together as as shown. Find the moment of inertia of the system about axis AB.

If it can be expressed as M R 2 × ( c d 3 π ) ÷ 2 MR^2\times({c}-\frac{d\sqrt{3}}{\pi})\div2 , find c + d c+d .


The answer is 14.

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3 solutions

K T
Jun 18, 2019

The points where the ring segments meet are distance R apart, so the angle of reach segment is 60° or π 3 \frac{π}{3} radians.

Moment of inertia L = r 2 d m L=\int r^2dm .

Note:

  • we count 4 half segments, each with angle running from 0 0 to π 6 \frac{π}{6}
  • r is the distance to the axis of rotation (the line AB), we express it as a function of the angle: r = R ( cos φ cos π 6 ) r=R(\cos φ-\cos\frac{π}{6}) .
  • the mass per angle then is d m d φ = 3 M π \frac{dm}{dφ} = \frac{3M}{π}

L = 4 0 π 6 R 2 ( cos φ cos π 6 ) 2 3 M π d φ L = 4\int_0^{\frac{π}{6}}R^2(\cos φ-\cos \frac{π}{6})^2 \frac{3M}{π}dφ

= 12 R 2 M π 0 π 6 cos 2 φ 3 cos φ + 3 4 d φ = \frac{12R^2M}{π}\int_0^{\frac{π}{6}} \cos^2 φ- \sqrt{3}\cos φ +\frac{3}{4} dφ

= 12 R 2 M π 0 π 6 1 2 cos 2 φ + 1 2 3 cos φ + 3 4 d φ = \frac{12R^2M}{π}\int_0^{\frac{π}{6}} \frac{1}{2}\cos 2φ + \frac{1}{2} - \sqrt{3}\cos φ +\frac{3}{4} dφ

= 12 R 2 M π ( 1 4 sin π 3 + π 12 3 sin π 6 + 3 4 π 6 ) = \frac{12R^2M}{π} (\frac{1}{4}\sin \frac{π}{3} + \frac{π}{12} - \sqrt{3} \sin\frac{π}{6} +\frac{3}{4} \frac{π}{6})

= 12 R 2 M π ( 5 π 24 3 8 3 ) = \frac{12R^2M}{π} (\frac{5π}{24}-\frac{3}{8}\sqrt{3})

= R 2 M 2 ( 5 9 3 π ) = \frac{R^2M}{2} (5  -\frac{9\sqrt{3}}{π})

5 + 9 = 14 5+9=\boxed{14}

Pulkit Deshmukh
Oct 2, 2015

I lost my points as I got trapped in that 'divided by 2' in the answer form that is written in the question...: (

Ankit Kumar Jain - 3 years, 3 months ago
Ayush Choubey
Oct 2, 2015

Sorry , I would not be able to post the whole solution but the sol. will surely answer the question asked

In the given fig . "c" is centre of mass of the portion of the ring. O is centre of the ring and we need to calculate "I(P)"

You can calculate it using the method to calculate that of a semicircular ring and it comes out to be - 3 R π \frac { 3R }{ \pi } = OC

I (O) - M(OC )^2 = I(C.O.M) ... Using parallel axis theorem .

I (P) = I (C.O.M) + M(CP)^2

where cp = oc - op = 3 R π \frac { 3R }{ \pi } - 3 R 2 \sqrt { 3 } \frac { R }{ 2 }

( OP is calculated using trignometry)

Now , since both portions are similar ,

Net Moment of Inertia = 2 * I(P)

giving c = 5 and d = 9

Answer = 14 \boxed {14}

I apologize if something is missing or unclear .

i would add a shortcut to calculate com. if you remember it for half ring then divide it into 3 parts equal and let it's cm be at distance x let mass=m(each part)

{mx+mxsin(30)+mx(sin30)}/3m=2R/pi

x=3R/pi

aryan goyat - 5 years, 2 months ago

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