- #1
Vykan12
- 38
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My textbook claims that we can describe the kinetic energy of a rigid body as follows:
K = 1/2 Mv^{2}_{cm} + 1/2 I_{cm}w^{2}
The first term makes complete sense. Earlier the book showed that we can model an entire body's translational motion (and momentum and gravitational energy) as though its entire mass were concentrated at its center of mass. Naturally this idea carries out to K = 1/2 Mv_{cm}^{2}.
I don't understand the I_{cm} aspect of this formula. If the object is rotating about an axis that doesn't pass through its center of mass, then we can only describe I_{cm} theoretically through the parallel axis theorem. However, the book derives this formula with the assumption that the object IS in fact rotating about its center of mass.
I can't help but think I'm conceptualizing rotation all wrong. If I throw an object up in the air, it only has 1 axis of rotation, not infinitely many, right? Like if I say an object is rotating about a point P, I can't also describe it as rotating about a point P' without changing my frame of reference.
K = 1/2 Mv^{2}_{cm} + 1/2 I_{cm}w^{2}
The first term makes complete sense. Earlier the book showed that we can model an entire body's translational motion (and momentum and gravitational energy) as though its entire mass were concentrated at its center of mass. Naturally this idea carries out to K = 1/2 Mv_{cm}^{2}.
I don't understand the I_{cm} aspect of this formula. If the object is rotating about an axis that doesn't pass through its center of mass, then we can only describe I_{cm} theoretically through the parallel axis theorem. However, the book derives this formula with the assumption that the object IS in fact rotating about its center of mass.
I can't help but think I'm conceptualizing rotation all wrong. If I throw an object up in the air, it only has 1 axis of rotation, not infinitely many, right? Like if I say an object is rotating about a point P, I can't also describe it as rotating about a point P' without changing my frame of reference.