Consider an object that has rotational inertia I about some arbitrary axis of ro
ID: 1480009 • Letter: C
Question
Consider an object that has rotational inertia I about some arbitrary axis of rotation. The radius of gyration of the object is the distance from this axis where all the object's inertia could be concentrated and still produce the same rotational inertia I about the axis. What is the radius of gyration of (a) a solid disk of radius R about an axis perpendicular to the plane of the disk and passing through its center, (b) a solid sphere of radius R about an axis through its center, and (c) a solid sphere of radius R about an axis tangent to the sphere?
Explanation / Answer
a)
For solid disk, I =0.5*m*R^2
If radius of gyration is k, then I can be written as,
I = m*K^2
equate both
0.5*m*R^2 = m*K^2
K = R * sqrt (0.5)
= 0.707*R
Answer: 0.707*R
b)
For solid sphere, I =(2/5)*m*R^2
If radius of gyration is k, then I can be written as,
I = m*K^2
equate both
(2/5)*m*R^2 = m*K^2
K = R * sqrt (2/5)
= 0.63*R
Answer: 0.63*R
c)
Throgh tangent, I = (2/5)*m*R^2 + m*R^2 = (7/5)*m*R^2
If radius of gyration is k, then I can be written as,
I = m*K^2
equate both
(7/5)*m*R^2 = m*K^2
K = R * sqrt (7/5)
= 1.18*R
Answer: 1.18*R