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[QUOTE]Originally posted by Joshua Bell: [QB] [QUOTE]Originally posted by Guardian 2000: [qb] Any system which causes the exhaust to reverse direction (even a forcefield) will be doing nothing more than "bouncing" the exhaust toward the front of the ship, canceling out the forward thrust and bringing the thrust total down to zero. Just because the exhaust products are now headed forward does not mean that the ship must now move backward. [/qb][/QUOTE]Hogwash. There are two ways to look at the problem, both of which are equivalent and give the same answer. ----- First, consider the ship and any assorted engines and thrust deflectors as a black box. The only things coming out of the black box is a stream of particles moving faster than the ship in the direction of travel. To conserve energy and momentum, the black box must be accelerating in the opposite direction. That's basic physics. ----- Second, consider the transfer of momentum at the level of particles, starting in the fusion drive. The fusion engine is imparting energy to the particles along random orientations which imparts no net momentum to the ship. Some of the particles bounce off the sides of the engine, but this happens in a symmetric fashion so there is no net transfer of energy (some of the particles hit the top accelerating the ship upwards and the particles reflect downwards; some of the particles hit the bottom, accelerating the ship downwards and the particles reflect upwards). Some of the particles hit the front of the engine, accelerating the ship forwards while the particles reflect backwards. The rest of the particles escape from the engine without a (net) transfering energy to the ship (i.e. they do, but it is offset by the particles that started off going forwards). The net result of this is that there is a stream of particles leaving the engine to the reverse while the ship is given a forwards momentum. Note that all of the particles that leave the engine must have a negative direction - and all particles must leave the engine eventually or it will clog up. Now this stream of particles impacts a thrust reverser and reflects forwards. This imparts a negative momentum to the ship. Note that ALL of the particles leaving the engine impact the thrust reverser and transfer negative momentum, while only particles which impacted the front of the engine imparted positive momentum. Thus, there is a net negative momentum imparted to the ship. If this were somehow balanced (i.e. the same number of particles impact the front of the engine as the impact the back) the ship would still experience a negative momentum increase. For example, if the initial momentum imparted to the particles was not omnidirectional (imagine you're running an ion drive which acts like a microscopic rail gun) then just imparting positive momentum to the particles (i.e. towards the front of the engine) necessarily imparts negative momentum to the ship. The particles then bounce off the front of the engine (a bit of positive momentum), then off the thrust diverter (a bit of negative momentum). Since the latter two particle bounces cancel each other out, you are left with the initial negative momentum from the particle generation. Both of these viewpoints (black box & watching particles) match, which is what you'd expect. ------ Your car/balloon/cardboard example is flawed. First, in most cases, the balloon is going to have only just enough energy to move the car without the cardboard in place (i.e. given air resistance and friction, it's not imparting 10x the energy to get the car going, probably only around 2x). Secondly, the air bouncing off the cardboard primarily goes off to the sides; it is not a unidirectional thrust diverter. More than likely, this reduces the available energy from 2x to 0.5x and thus the car doesn't move. If you used a styrofoam cup as the diverter to direct the flow unidirectionally, I bet it would work just fine. ----- Lastly, jet airliners use thrust diverters to slow the plane upon landing. This is practical evidence that the darn things do work. [/QB][/QUOTE]
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