Question

Potential Energy

Answer 1

http://www.youtube.com/watch?v=oAc4TP0MXBk
at 47.00

Answer 2

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

Answer 3

@JamesJ Please share some insight :)

Answer 4

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

Answer 5

watching ... one sec

Answer 6

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 \( \omega \).

Answer 7

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

Answer 8

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).

Answer 9

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

Answer 10

Intriguing stuff!

Answer 11

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

Answer 12

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

Answer 13

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