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Explaining the Fluid Drive V3 with two astronauts and a steel ball (previously three kids, an umbrella, a skateboard and batman)


Tongue in cheek explanation of how the Fluid Drive proposal works using a spaceship, astronauts and Batman (Adam West version)


In fig 1 we have a spaceship in microgravity, inside are two astronauts (A and B) and a steel ball.

Fig 1


Astronaut A and astronaut B are at the forward and rear end of the spacecraft, if astronaut B pushes the steel ball at astronaut A with force F1 (red arrow) he will receive a counter force F2 (blue arrow) that will be transferred to the spacecraft



Fig 2


As the steel ball travels towards astronaut A with velocity V2 (fig 2) the spaceship is traveling in the +X direction at velocity V1.



Fig 3



The steel ball collides with astronaut A (no doubt he feels the force that is transferred to the spaceship) as their momentums are equal (momentum = velocity x mass) system (spaceship and all inside) returns to original velocity 0.



Fig 4


If astronaut A pushes the steel ball back at astronaut B (fig 4), when the ball collides with astronaut B the result will be the same (the spaceships velocity returns to 0), they can play all day but all the spaceship will do is wobble.


What happens if an air brake is activates when the steel ball is traveling from B to A?


Fig 5


When speeding towards astronaut A, the steel ball activates an air brake (fig 5) that increases the steel ball’s drag reducing its velocity from V2 to V3i, the spacecrafts V1 velocity is not affected (not very much).


Fig 6



When the steel ball collides with astronaut A, the spaceship’s momentum P1 is almost identical to instant illustrated in fig 3, but as the steel ball’s velocity has decreased from V3i to V3f its momentum p3 is less than momentum p1 and the force excreted by the steel ball on astronaut A (and the spaceship) is not sufficient to return the spaceship’s original velocity therefore it will gain velocity in the +X direction.


Astronaut A can return the steel ball to astronaut B with a very slight push (it does not matter that it takes longer to travel the length of the spaceship) so astronaut B repeats this cycle and the spaceship will gain velocity with every cycle.



At this point I generally get the following comment:


This will not work because all the impulse of the mass (steel ball), that you "get rid of" by using airbrakes, is actually transferred to the air inside the cylinder which pushes against the wall of the cylinder and accelerates it. It is all a closed system, so there will be no change in impulse and no acceleration at all.


This idea that the airbrake pushes against the AIR and the AIR pushes against the rear wall of the cylinder is illustrated in fig 6a.


Fig 6a


But that is not really how drag works, the airbrake does not push against an “AIR”, it pushes (collides) against individual fast moving gas molecules (fig 6b 1), for every collision of molecule against air brake there are two equal and opposite forces (no problem with Newton’s Laws), and the molecules that have been hurled in the general direction of the rear wall of the cylinder encounter billions and billions of fast moving molecules (fig 6b 2), as air has a tendency to expand in every direction to fill the container (equalizing local differences in air pressure) some of the force is apparently diverted towards the other walls of the container(1).



Fig 6b


There are also some that say when the airbrake collides with the air, the air in front of the airbrake is compressed (fig26c 1) and that it will travel to the rear wall of the cylinder like a Kamehameha (2). However as one of the basic characteristic of a gas is its tendency to equalize its pressure when contained (we do not see a tendency for higher pressures inside a container at rest) part of the momentum will be diverted to the other walls of the cylinder.

Fig 26c


(1) Actually I do not really know with absolute certainty how the air molecules behave for I cannot directly observe them (I do not have a Schlieren set up available), I can expect air to behave in a certain manner because of the kinetic theory of gases. But what I do know is what I can observe, and the experiment described in , and the experiment described in a DIY test (figs 13 to 21) work as described.


(2) Totally unnecessary dragon ball reference.


A step by step explanation of the principles involved (for the serious minded, no kids, no skateboard, no batman, no puns).


The batman postulation


Chris from Brock U. had the following comment: Would the application of the air brake (fig 9 B) not create air movement similar to that created by a propeller (fig 9 B)?


Fig 9


I think not, picture yourself as batman (Adam West incarnation) trapped in a box with a wheeled cart speeding towards him.


If the speeding cart is slowed and stopped by a propeller (fig 9 B) you (you’re batman remember) WILL feel a strong breeze to say the least but if the speeding cart is slowed and stopped by a parachute (like a drag racing car) you will not feel the same “breeze”


A DIY test


This idea can be tested (or disproved) with a very simple test setup.


Fig 13


We need a box on a near frictionless surface, inside the box a test car and a mechanism (in this example a compressed spring)


Fig 14


When the spring is released the box (M1) will gain a velocity (V1) in the +X direction and the car will gain a velocity (V2) in the –X direction. borrar


Fig 15


As they travel in opposite directions their velocities (V1 and V2) remain approximately constant.



Fig 16


When the car collides with the box’s inner wall, it’s momentum (mass X velocity) will be sufficient to stop the box that will return to its original velocity 0.


This test must be repeated various times

If the box final velocity is different from 0 (however slightly) it means the test bed is flawed and the resulting velocity must be subtracted from any results obtained.


Fig 17

We can now repeat the experiment with an air brake attached to the test car.


Fig 18

When the spring is released the box (M1) will gain a velocity (V1) in the +X direction and the car will gain a velocity (V2) in the –X direction.

Fig 19


Fig 20

As the test car moves in the –X direction (Figs 19and 20) the addition of the air brake will (should) decreases it’s velocity (V2i > V2a > V2b > V2f)



Fig 21



When the test car collides with the box, if the drag produced by the air brake on the test car does not affect the box, then the test car’s momentum (V2f x cars mass) will have decreased while the box’s momentum will have remained the same therefore the box should have a noticeable +X final velocity


This has been a Tongue-in-cheek description of the idea, the serious minded please see step by step (no kids, no skateboard, no batman, no puns)



See description of this effect for propulsion here.


Main Page, Other versions, testing the principle, testing version 2


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