Rotation 1 Question 17

20. Let I be the moment of inertia of a uniform square plate about an axis AB that passes through its centre and is parallel to two of its sides. CD is a line in the plane of the plate that passes through the centre of the plate and makes an angle θ with AB. The moment of inertia of the plate about the axis CD is then equal to

(a) I

(b) Isin2θ

(c) Icos2θ

(d) Icos2(θ/2)

(1998,2M)

Objective Question II (One or more correct option)

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Answer:

Correct Answer: 20. (a)

Solution:

  1. ABAB and CDCD

From symmetry IAB=IAB

and

ICD=ICD

From theorem of perpendicular axes,

IZZ=IAB+IAB=ICD+ICD=2IAB=2ICDIAB=ICD

Alternate

The relation between IAB and ICD should be true for all values of θ

At

θ=0,ICD=IAB

Similarly, at θ=π/2,ICD=IAB (by symmetry)

Keeping these things in mind, only option (a) is correct.



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