Two portions of a thin ring each of mass M and and radii 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 − π d 3 ) ÷ 2 , find c + d .
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I lost my points as I got trapped in that 'divided by 2' in the answer form that is written in the question...: (
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 - π 3 R = 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 = π 3 R - 3 2 R
( 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 = 1 4
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
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The points where the ring segments meet are distance R apart, so the angle of reach segment is 60° or 3 π radians.
Moment of inertia L = ∫ r 2 d m .
Note:
L = 4 ∫ 0 6 π R 2 ( cos φ − cos 6 π ) 2 π 3 M d φ
= π 1 2 R 2 M ∫ 0 6 π cos 2 φ − 3 cos φ + 4 3 d φ
= π 1 2 R 2 M ∫ 0 6 π 2 1 cos 2 φ + 2 1 − 3 cos φ + 4 3 d φ
= π 1 2 R 2 M ( 4 1 sin 3 π + 1 2 π − 3 sin 6 π + 4 3 6 π )
= π 1 2 R 2 M ( 2 4 5 π − 8 3 3 )
= 2 R 2 M ( 5 − π 9 3 )
5 + 9 = 1 4