# Rotational Kinetic Energy

#### Victorian91

1. The problem statement, all variables and given/known data
A uniform rod AB is hinged smoothly at A. The length L of the rod is 50cm. Determine
a) The angular velocity,

of the rod when it makes an angle (Theta) =30o with the verticle.

( Moment of inertia of rod about axis through one end and perpendicular to the rod = 1/3 mL2 ; g =9.8m s-2

2. Relevant equations
Ki + Ui = Kf + Uf

3. The attempt at a solution

0 + mghi = 1/2 Iw2 + mghf

Where hi = 1/2 L

and hf = 1/2 L ( 1-cos Theta )

Then plugging in the information given,
Angular velocity = 7.1 rad s-1

When the rod is vertical, let the horizontal line which passes through the center of the rod be the reference line where h=0 and U=0.

Can anybody tell me why the final potential energy is not just 1/2 L cos Theta.

Or is it 1/2 L ( 1-cos Theta ) ?

Thanks...

#### tiny-tim

Homework Helper
Can anybody tell me why the final potential energy is not just 1/2 L cos Theta.

Or is it 1/2 L ( 1-cos Theta ) ?
Hi Victorian91! (have a theta: θ )

potential energy can be measured from anywhere

it's your choice …

if you chose to measure it from θ = 0, then you have make sure that it's 0 at θ = 0 ### The Physics Forums Way

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