# I Rotational Potential Energy?

1. Oct 6, 2016

### FallenApple

So masses on springs store potential energy. Height in a gravational field store potential energy for the mass there.

So why isn't there a potential energy stored inside rotating objects? Surely there are ways to translate the rotational energy to kinetic. Its kinda like a spring. If a set down a fast spinning object, it will roll forward slowing its rotation down and speeding up its linear motion. One type of energy is transfered to another.

2. Oct 6, 2016

### drvrm

rotations are described by 'kinetic' state - a change of configuration so why we seek a potential energy-
potential energy is manifest in the position of bodies in a 'field'
if one is sitting at the seventh floor of a building he has a potential to do work if he falls to the ground in earth's gravitational field- its not due to inertia of rest or motion.
if the person is spinning as well as sitting on the sae height -what additional potential he developes- added energy is best estimated by his rotational energy which is 'kinetic ' in character.

3. Oct 6, 2016

### Staff: Mentor

I don't understand. It is already kinetic.

4. Oct 6, 2016

### CWatters

What Dale said. The energy rotational energy stored in a spinning mass _is_ kinetic energy. No need to "convert it".

You can calculate it as..

E = 0.5 I ω2

where
I is the Moment of Inertia (analogous to mass in a linear system)
ω is the angular velocity (analogous to velocity in a linear system)

See torsion spring. https://en.wikipedia.org/wiki/Torsion_spring

5. Oct 6, 2016

### CWatters

That might appear to be the case but you need to think about the whole system including the planet you are doing the experiment on.

If you spin up a flywheel on the surface of the earth you affect the rotation of the planet (Newtons laws). There is no net change in the angular momentum of the combined planet/flywheel system. That's because there has been no external force applied to the system.