A rather esoteric error has been made by inventors who weighed a spinning
wheel or vibrating device, and found that it seemed to weigh less when
running than when stationary.
Dean's machine could sit on a floor or table, and slowly crawl along. He even claimed it could move up a very slight incline. So "with a little more work" he hoped to make one that could be turned on its side, and levitate itself up away from the floor. That never happened.
What is happening is due to "stiction", the "stick and slip" phenomenon of friction. The coefficient of static friction is generally larger than the coefficient of sliding friction. A standard physics demonstration, the tablecloth pull, uses objects arranged on a tablecloth. When the cloth is pulled slowly, the objects move along with it, since the maximum value of static friction has not been reached and sliding doesn't initiate. But when the cloth is yanked quickly, the threshold force is quickly exceeded, the friction coefficient drops to a lower value and the cloth comes away leaving the objects on the table nearly where they were originally. The whole thing happens before the objects on the cloth can move very far. This is called the "table cloth effect", and it obeys classical physics completely.
Yet, apparently unaware of these well-known physics principles and engineering applications, people even today are still inventing, patenting and proclaiming "internal propulsion engines" and "reactionless drives". Many claim they are violating Newton's third law. Sometimes they even invent new theoretical physics to account for their imagined violations of physics. Among these are Robert L. Cook's inertial propulsion system US patent 4,238,968, Dr. Gennady Shipov's universal propulsion system, 6,347,766, and James Woodward's theoretical proposal of a reactionless propulsion system, US patent 5,280,864.
Jerry Pournelle has a good account of Dean's device, and makes a suggestion for testing such things. Invariably inventors test them on surfaces or rails, leading one to suspect that friction is doing the dirty work. I haven't ever seen one tested on an air suspension table, to reduce that possibility. Also, anything that rotates may have a "fan effect" causing movement by pushing against air. So Jerry suggests reducing the friction by suspending the device from wires. Also, one could do the experiment in a vacuum. Actually, it would be sufficient to enclose all moving parts in a box that allowed no communication between the internal air and the external air. If, under these conditions, the device swings to one side when running and returns to center when stopped, then, Jerry says, he might get interested. But inventors do not do this, or anything close to it. Hmm.... .
|Harry Bull tests his device.||The shifting weights.|
Even with this arrangement, self-deception can occur, as in Henry Bull's impulse engine of 1935. You can read about it in Popular Science Monthly, Jan 1935, p. 27: Harry W. Bull: Reaction Motor. His device was in an enclosed box, and suspended from wires as a pendulum. Inside the box two weights were driven by electromagnets, one weight making an inelatic impact with a spring, the other making a nearly elastic metal-to-metal impact. When running, the box containing the device moved to the side. Why? Due to the asymmetric motion inside the box, the center of mass of the box and its contents shifts relative to the box. But the center of mass must still remain where it was before (relative to the laboratory). So the box moves aside, while its center of mass stays put. Newton's laws were working properly, as they always do.
I can't help wondering why these inventors who circulate video clips of their devices moving along a flat surface don't take it another step. Let the device move around a circular track (perhaps slightly "banked") and make a perpetual motion machine. Then they might be motivated to ask whether their "physics defiant" device is also violating conservation of energy, which they could test by measuring the power input to the rotating weights of their device. I confidently predict that energy conservation will be found to be working properly.