# Physics Mobile / Ruler

1. Dec 16, 2008

### teedoff087

See http://technology.todaysbigthing.com/2008/12/16" [Broken].

Can someone explain this?

Last edited by a moderator: May 3, 2017
2. Dec 16, 2008

### olgranpappy

yes, I believe so. I couldn't hear the sound on the video (my computer's problem) so I'm not sure I'm explaining exactly the right thing. But here is a try.

It should be easy to see how the spindels cause the whole apparatus to move in the direction opposite the ruler moves. Then, the ruler moves across the top of the big wheel. If this were normal rolling motion the ruler would move at twice the speed of the center of mass of the big whell this is because rolling is a combination of translation and rotation.

The actual motion of the apparatus is again a combination of translation and rotation, but because of the spindels the translational motion is not in the same direction as the rotational motion of the top point of the wheel thus the effects tend to cancel reducing the distance the ruler travels rather than increasing it as in normal rolling motion.

Last edited by a moderator: May 3, 2017
3. Dec 16, 2008

### teedoff087

Thank you. That's very helpful. Some members on another board I frequent were confused so I figured I would find out for them (and of course myself).