
Concept explainers
(a)
Percentage difference between the moment of inertia in two cases.
(a)

Explanation of Solution
Given:
Diameter of each sphere
Radius of sphere
Length of the rod
Mass of each sphere
Mass of each rod
Moment of inertia of each sphere about its center of mass
Moment of inertia of each sphere about axis of rotation
Moment of inertia of rod about axis of rotation
Distance of center of each sphere from axis of rotation
Moment of inertia of the system when spheres are treated as particle
Moment of inertia of the system
Percentage difference
Formula Used:
Moment of inertia of each sphere about its center of mass is given as
Moment of inertia of rod about its center of mass is given as
According to parallel axis theorem, moment of inertia about the axis of rotation is given as
Where, d is the distance between center of mass and axis of rotation.
Percentage difference in the moment of inertia is given as
Calculation:
Case 1:
Consider the two spheres as point particles and mass of the rod negligible.
Distance of center of each sphere from axis of rotation is given as
Moment of inertia of the system is given as
Case 2:
Moment of inertia of each sphere about its center of mass is given as
Using parallel axis theorem, moment of inertia of each sphere about axis of rotation is given as
Moment of inertia of rod about its center of mass is given as
Moment of inertia of the system is given as
Percentage difference in the moment of inertia is given as
Conclusion:
Hence, Percentage difference in the moment of inertia is
(b)
The moment of inertia of system will change if solid sphere is replaced with hollow shell.
(b)

Explanation of Solution
Given:
Diameter of each sphere
Radius of sphere
Length of the rod
Mass of each sphere
Mass of each rod
Moment of inertia of each sphere about its center of mass
Moment of inertia of each sphere about axis of rotation
Moment of inertia of rod about axis of rotation
Distance of center of each sphere from axis of rotation
Moment of inertia of the system
Formula Used:
Moment of inertia of each sphere about its center of mass is given as
Moment of inertia of rod about its center of mass is given as
According to parallel axis theorem, Moment of inertia about the axis of rotation is given as
Where, d is the distance between center of mass and axis of rotation.
Calculation:
Moment of inertia of each hollow sphere about its center of mass is given as
Using parallel axis theorem, moment of inertia of each hollow sphere about axis of rotation is given as
Moment of inertia of rod about its center of mass is given as
Moment of inertia of the system is given as
Conclusion:
Hence, the moment of inertia of the system increases.
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