- #1
unscientific
- 1,734
- 13
Homework Statement
Find the moment of intertia of a pendulum, consisting of a disc free to spin attached to a rod that is hinged at one end.
Homework Equations
Moment of intertia of rod hinged at end = (1/3)Ml2
Moment of intertia of disc = (1/2)mR2 + ml2
The Attempt at a Solution
Why does the answer disregard the part moment of intertia of the disc (1/2)mR2 that is spinning on its own axis, and only taking into account the ml2?
Here's what the answer wrote:
"If the disk is not fixed to the rod, then it will not rotate as the pendulum oscillates.
Therefore it does not contribute to the moment of inertia. Notice that the pendulum is no
longer a rigid body. So the total moment of inertia is only due to the rod and the disk
treated as a point like object."
I got completely lost by the first sentence. Why does the disc not contribute to the moment of inertia when it is spinning? I thought the idea behind moment of inertia is linked to the rotational kinetic energy it possesses? Oscillating a spinning disc does not rob it of its kinetic energy!