The length of a pendulum that is equivalent to a rocking hemisphere

[chriskh]

Homework Statement

A solid sphere is cut in half and a homogeneous hemisphere of radius r and mass M is set upon a table(with its flat side up). The surface of the table is perfectly rough. The hemisphere rocks back and forth with small amplitude excursions from equilibrium. What is the length of an equivalent simple pendulum? Justify approximations. Note that the center of mass of a hemisphere is at a distance 3r/8 below the center of the sphere.

Homework Equations

The Attempt at a Solution

 

[PeroK]

Can you give it your best shot?

 

[chriskh]

The answer should be 1.73r

I can't find what's wrong with my solution.

 

[PeroK]

I'm not sure about your derivation of either $T$ or $V$. Can you justify those?

You may need to do some serious geometry!

 

[J Hann]

I think you need 3/8 in the denominator instead of 5/8.

 

[PeroK]

I think you need 3/8 in the denominator instead of 5/8.

There's more than that is not right.