Nature's Laws Not Negotiable
There is something in the human brain that seems not to accept the physical laws governing the world. This leads to what has been called the "Oh, yes" effect, as people remember their high-school physics lessons most clearly after something has gone wrong.
A common example is the person who pounds at the bottom of a catsup bottle, hoping to drive a few more drops out of the end. Newton's laws of motion state that an object at rest will stay at rest unless acted upon by an exterior force. Thus, when the catsup bottle is struck, the bottle moves forward, but the catsup wants to stay where it is. Pounding on the bottle's bottom actually moves the catsup further backward into the bottle.
The flip side of this law states that when an object is in motion it will stay in motion until meeting with a force that alters its direction. Thus, the correct way to empty a catsup bottle, according to Newton, is to hold the bottle well overhead and swing it rapidly toward the table, stopping it inches above the catsup's desired destination. This way, although the bottle stops, the momentum of the catsup inside carries it forward and out the spout. (The technique probably should be first mastered at an outdoor barbecue rather than in a formal restaurant. )
Inertia (the tendency of the catsup to stay in place while at rest) and momentum (the tendency for the catsup to continue moving in a straight line once it has been set into motion) are properties with which we deal every day. These properties explain why it takes many miles to stop an ocean-going tanker once it is underway, or why it is relatively easy to drive a nail into a large log, but difficult to drive one through a small unanchored board.
Anything with mass has inertia and momentum. (Astronauts moving about in the orbiting space shuttle are dealing with mass, but on earth we can mentally substitute weight for mass). It is common in older houses to hear pipes rattle in the walls when a water tap is turned off. The moving water has gained momentum and--in the absence of air-filled shock absorbers--when it is abruptly stopped, it transfers the energy to its surroundings. Garden hoses can be seen to writhe when a nozzle at the end is suddenly closed. Depending on the amount of liquid being transported, pressures within the conduit can become significant.
Moving water has given me some very pointed reminder lessons about Newton's laws. One summer a field party I was with wanted to bring some water down from a mountain lake into the camping area. We had a large supply of one-inch PVC plastic plumbing pipe on hand, and it was a simple matter to lay a pipeline from the lake into the camp, installing shutoff valves at both ends. The lake was about 100 feet above the level of the camp and a couple of hundred yards distant. This provided a good head and lots of pressure, as we found out spectacularly the first time we abruptly closed the lower valve. Stopping that volume of water in its tracks asked too much of the pipe, and about 20 feet of its lower end ripped open. Afterward, we were careful to close the lower shutoff valve slowly.
Thus do people learn again--and again--about the little surprises of momentum. Still, I know what I am likely to do to get the last bit out of the next catsup bottle.
