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Rod released on a hinge

  • #1

Homework Statement



A massless rod length L has a small mass m attached to the center and another mass m attached at one end. On the opposite end, the rod is hinged to a frictionless hinge. The rod is released from rest at a horizontal position and swings down. What is the angular velocity as it swings through its lowest (vertical) point? Solve in terms of g and L.

Homework Equations



use moment of inertia, energy conservation

The Attempt at a Solution



I determined the total moment of inertia of the two masses to be (5/4)mL^2. I know that initial potential energy is 2mgL (setting bottom point of swing as zero). At the bottom, the mass in the middle has PE mgL, the system has KEr of (.5)Iw^2, and both masses have a translational kinetic energy. All of this must sum to the intial 2mgL? I think I am just missing a step in the algebra.
 

Answers and Replies

  • #2
Doc Al
Mentor
44,892
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The position of the mass in the middle is L/2 from one end, not L (redo your PE terms). Also, you can treat the system as being in pure rotation about the hinge.
 

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