Reactionless drives
and "internal propulsion".

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.

It's that old nemesis of perpetual motion machine inventors—friction. Back in the heyday of spiritualism, some gullible people were taken in by the classic "five finger lift" in which five persons lift another, using only one finger each. The trick depends on human physiology. When five people try to lift a rather cumbersome and non-rigid object like a person, their efforts are uncoordinated. But when they are told to take a deep breath, count to three and then lift in unison, the task is easy. But spiritualists thought it was due to some sort of spiritual or psychic "energy". Spiritualists did tests with an industrial mechanical scale large enough for six people to stand on. They found that the weight registered by the scales after the lift was slightly less than that before the lift. Any first-year physics or chemistry student used to working with mechanical balance scales would know the reason. Even the best scales have residual mechanical friction and hysteresis. So, when balanced, then moved off balance, the scale doesn't return exactly to its original balance point. Experienced workers compensate for this by tapping the scale's frame till the truer balance is achieved. When the five finger lift was done on the industrial platform scale, the act of lifing the person suddenly unbalanced the scale toward a larger weight, and when the lift was complete, the scale slowly returned toward its balance point, but stopped short of that and did not register the correct weight. This is just one of the things that can mislead inexperienced users of mechanical scales.

Balance scales and even electronic balances can also be fooled by vibrations, due to mechanical "stiction" (the "stick and slip" phenomenon of friction). The scale itself is affected by nonlinear phenomena in its mechanism, and these can often display resonance peaks, dependent on the frequency of the vibrations. So a running motor on a balance scale may indeed seem to weigh less when it's running than when it isn't. That has fooled many people, and is one of the reasons for the strange results when Norman Dean demonstrated his "reactionless" drive of the early 1960s.

Norman L. Dean demonstrates
the Dean Drive in 1961
U.S. Patent 2,886,976

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.

The classic tablecloth pull.
The same principle has industrial applications, in vibratory conveyors, or "slipstick conveyors" for transporting granular or bulk materials over relatively short distances. A trough conveyor is caused to vibrate asymmetrically, moving slowly in one direction, then more quickly in the other direction, doing this cyclically. The material on the trough moves in one direction continuously. In some applications this has advantages over belt or roller conveyors. One can demonstrate this principle using a shallow box with a coin in it. Hold the box in one hand, and with the finger of your other hand tap one end of the box repeatedly. In this way you can cause the coin to move continuanally up a slight incline. Rest assured that you violate no physics laws when you do this.

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.

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