Description: A meter stick is supported on the two forefingers. The fingers are moved slowly together and end up at the center of the stick.

Start with the arms apart and move them together to end up in front of the body. Start with the fingers at different distances from the center of the stick. Try holding one arm directly in front of the body and moving only the other arm. The alternation between slipping of the stick on the moving finger vs the stationary finger is more evident this way.

Load the meter stick at one end with a 200 gm weight. This will move the center of gravity to approx 22 cm from one end.

Use the plexiglas cylinder and the cork covered piece of wood instead of the fingers to show that this also works if the coefficients of friction of the two supports are very different.

Demonstrate the requirement that the fingers must be moved 'slowly'.

We have an 8 foot length of 2 by 2 which may be used in place of the meter stick and works well. A 1 kg weight on the end moves the center of gravity to 70 cm from that end.

Here is how Julius Sumner Miller presents this demonstration (Demonstrations in Physics pages 16 and 17):

"Meter stick on two fingers

I. Support Symmetric
A meter stick is supported horizontally on two fingers symmetrically placed. The fingers are pushed toward each other.

1. When at rest what force is supplied by each finger?
2. When the fingers are pushed towards each other what forces now exist at each finger?
3. What condition must be met for the fingers to experience identical motion?
4. What now is the question to ask? And how is it answered? Why is this the answer?
5. Suppose the fingers are replaced by two supports of enequal roughness, say an iron rod and a wooden stick or a rubber rod. What now?
6. Can you conjure up a situation where, with symmetric supports to begin with, the supports will not meet in the middle?
7. Suppose in all of this the meter stick is not uniform. What now will happen? Consider doing it with a baseball bat.
8. Conjecture on what will happen if the fingers are moved away from each other.

1. Consider each of the questions under I.
2. The 'leading question', the principal inquiry, in this exercise is clearly this: In I the fingers (supports) meet in the middle. This is to be expected! In II the 'feeling' is that the stick will tip. But it does not! State in ordinary language for those unschooled in physics why it does not.
3. At times in the experiment an observer claims that the fingers (supports) are not being moved or even pushed. Argue this away.
4. These exercises were done with the stick horizontal. Explore the matter with the stick inclined somewhat, that is, one support a bit higher than the other.
5. Suppose you make worse the conditions in both cases by loading the stick somewhere. What now obtains?
6. Conjecture on the following: Two motors with long shafts reside side by side. Across the two shafts a plank or 2x4 is placed. The shafts are now turned by the running motors. What will happen to the plank?"

Cautions:

Concepts demonstrated: center of gravity, dependence of friction on the normal force, balance of torques

Setup Time: Short

Difficulty/Commitment: Straightforward

Visibility: Visible

Related demonstrations:

References: PIRA 1J10.20
Video Encyclopedia 04-15
Sutton M-50; DaR M-478; Taylor 1.23 p47; Ehrlich(1) D9 p49; Miller p16; Kutliroff 47 p106; Freier Fig 10.8 p100; Steinhaus p140