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I am rather perplexed by the

*solution*to the following problem.

Particles of mass M1 and M3 are fastened to the ends of a light rod having a length l. A bead of mass M2 is free to slide along the rod between M1 and M3. Point

*p*is the center of mass of M1 and M3,

**not including M2**.

**I**is the moment of inertia of the M1, M3 rod arrangement about an axis perpendicular to the rod and passing through

*p*. All motion is considered in a plane. :yuck:

Now the translational aspect of the KE is very simple. But it was the rotational part that had me confused.

In my humble opinion we cannot use

**I**and

*p*coz as we add M2 the center of mass changes.

However in the answer for the rot. KE they have

[tex]\frac{1}{2}[/tex]

**I**@theta/@dt + [tex]\frac{1}{2}[/tex] M2[...............................................]

The thing that bothers me is that surely with the addition of M2, the value of

*p*and hence

**I**would change. So how can they treat the other masses independently from the third?????

I hope I have been fairly clear and am not being too silly.

Thanks guys!