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A uniform rod of length L and mass M is pivoted about one end. Attached to the rod is a mass M at its midpoint and at the end opposite the pivot is another mass M. What is the moment of inertia of this configuration about the pivot?

Question

A uniform rod of length L and mass M is pivoted about one end. Attached to the rod is a mass M at its midpoint and at the end opposite the pivot is another mass M. What is the moment of inertia of this configuration about the pivot?

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Solution

The moment of inertia, I, of a system of particles is given by the sum of the products of the mass of each particle and the square of its distance from the axis of rotation.

  1. For the rod: The moment of inertia of a rod about an end is given by (1/3)ML².

  2. For the mass at the midpoint: The moment of inertia of a point mass is given by MR², where R is the distance from the axis of rotation. Here, R is L/2 (since the mass is at the midpoint of the rod), so the moment of inertia is M(L/2)² = (1/4)ML².

  3. For the mass at the end: Again, the moment of inertia of a point mass is given by MR². Here, R is L (since the mass is at the end of the rod), so the moment of inertia is M(L)² = ML².

Adding these all together, the total moment of inertia of the system is (1/3)ML² + (1/4)ML² + ML² = (1 + 1/3 + 1/4)ML² = (1.58)ML².

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