M-D8: Inertia demonstration
 
PIRA: Unknown 

An impressive demonstration when it is successfully carried out.

Equipment: wooden lead pencil squared off at one end, letter size sheet of paper (16 or 20 substance bond or second sheet) with no burr from cutting or binding along the edge next to the pencil. Use one or more of the pencils from the equipotential lab located on a cart in Room 87.

Procedure: "Find a level spot on the table 6 to 7 inches back from the edge where the pencil can be made to stand stably on the squared end. Place the paper on the table so that when the pencil is set on the paper very near the center of the paper's smooth edge, the pencil will stand over the selected spot on the table with part of the paper hanging off the table. Finding the spot and setting the pencil up on the paper requires a bit of patience. I find it best to find the spot before lecture and make reference marks on the table from which I can locate the right place when needed in lecture. Do not mark the spot itself; the material left by the marking device will probably destroy the level condition. The proportion of paper over the table to that hanging off is about 2:1, though that hanging off can be increased a little. The paper must lie flat under the pencil, and there must be enough hanging off to snatch successfully.

"The snatch is accomplished in the following way. Lick the middle three fingers of the hand from the last joint outwards. Spread out well the fingers of the hand. Stand back a bit, place the forearm horizontal at about table-top level pointing with the hand toward the paper. Choose the distance of the hand from the table so that when the hand is brought down to engage the paper the finger tips will not strike the table. It is best not to flex the wrist rather than the elbow. Raise the hand and bring it down on the overhanging paper as rapidly as you can, --fingers wet and outstretched, swiveling largely at the wrist. The paper will cling to the wet fingers, sweep down to the floor and leave the pencil standing. This usually brings great applause. Note that the outstretched fingers are necessary to avoid windage from a closed hand which will blow the pencil over. The wet finger technique is the only method I have seen for getting sufficient frictional contact with the paper. While success can be achieved by swiveling at the elbow, there is more windage than in the case of swiveling at the wrist ¾ at least, I am not as consistently successful with the elbow method.

"Do not, please, give the often given explanation that the paper is out from under the pencil before the pencil can move. The pencil does move, but the design of the experiment has reduced the motion to an acceptable level which does not move the pencil out of its narrow stable regime. I recommend an explanation along the lines given in M-D6.

"Usually in a case like this I take time after the experiment to comment that this is a nice parlor trick that some might like to try sometime. Then I review concisely the important points in the experiment design as I have done here at the beginning." ¾Percy Carr (1970)