M-EngMom10: Conservation of energy; example: pile driver
 
PIRA: 1M10.20 

Equipment: laboratory pile driver, block of wood, nails, vernier caliper located at Set 9, Cabinet 1, Shelf 1 (although some are in B53 if necessary).

Procedure: The block of wood is best if 2 x 4 and about 4 inches long. Before lecture drill in the end of this block a few holes perhaps a half-inch deep, preferably on a drill press so that the direction is perpendicular to the base. The size of hole should be such that the nails to be used can be forced into them by hand or with light tapping of a hammer. The hole should be parallel to the grain if anything like quantitative results are to be obtained. When nails penetrate wood across the grain they stop at the end of each hammer blow partly penetrating a layer of lignin because it is so much harder than the cellulose layers in between. Drill the holes in the cellulosic layer (that is where they usually will go anyway).

In the lecture press, or lightly hammer, a nail into a drill hole and measure with the caliper the position of the nail head referred to top or bottom of the block. Latch the pile driver to the support at the top and place the block with nail under the pile driver. Measure the position of the driver with respect to a fixed reference (not the top of the nail). Release the pile driver which drives the nail. Measure the position of the driver referred to the same fixed reference and calculate the distance of the fall. If the driver does not have the mass marked on it, determine its mass, and calculate the loss of potential energy in its fall. Measure the new position of the nail head, calculate its penetration distance for that blow. Then calculate the mean force of the driver on the nail head during the blow.

If you care to repeat a few times, you can show how the mean force of penetration increases as the nail gets farther into the wood.

Warning: If one attempts to drive a large nail in hard wood with too small a hammer, all the kinetic energy of the hammer will be spent in compressing the nail before the force on the nail reaches that required for penetration into the wood. In such a case the elastic energy stored in the nail is returned as kinetic energy to the hammer, which flies back with amazing speed, often into the face of the person using the hammer.