## arcticf0x 3 years ago Potential Energy

1. arcticf0x
2. arcticf0x

Why the time difference? Its not angle, friction.. what is it?

3. arcticf0x

@JamesJ Please share some insight :)

4. arcticf0x

In that experiment, there is a difference in his prediction time and actual time. Why is that, as WL asks about it.

5. JamesJ

watching ... one sec

6. JamesJ

Ok. The difference between the two scenarios is whether or not the object oscillating has angular momentum. In the first case--with the air track--the bob has no intrinsic angular momentum and the situation can be analyzed with just 'translational kinematics'. In the second case, the ball does rotate about an internal axis, it has non-zero moment of inertia and hence it has non-zero angular momentum. Hence there is rotational kinetic energy $RKE = \frac{1}{2}I\omega^2$ and as the ball moves gravity and friction does work on the ball to make it change angular momentum $$\omega$$.

7. arcticf0x

Oh ok, so its the ball's inertia and rotaion! Does that affect his prediction time aswell?

8. JamesJ

So work is done on the ball to make it change its angular velocity, and that work takes energy away from the other forms of mechanical energy: translational KE and gravitational PE. yes, it most definitely impacts the physics, so the analysis he did on the board was wrong (and of course he knows it; he's making a point).

9. arcticf0x

Oh ok, so he purposely didnt take that work into account! Cool. But hey, does that roation and initial momentum affect his prediction time?

10. arcticf0x

Intriguing stuff!

11. JamesJ

the fact that the object does rotate about its own axis definitely effects the period of oscillation

12. arcticf0x

Yes indeed! I got the point(s), thanks for your time James!

13. pastory

p.e math=mgh m=mass g=gravitation h=height