Self Propulsion: Applications of Newton’s Law and Momentum Conservation: For time immemorial people have been cooking up schemes for low energy propulsion. Of course, we believe that whatever is designed better be compatible with Newton's Laws and the Law of Conservation of Momentum (which is actually a consequence of Newton's Second and Third Laws).Answer questions detailed in (a) through (d) by referring to the diagrams that follow. (a) A lazy fisherman turns on a battery operated fan and blows air onto the  sail of his boat. Will he go anywhere? If he moves, what will his direction be?  Explain. (b)A clever child is dangling a large magnet out in front of her wagon. It attr acts a smaller magnet that she has attached to the front of her cart. Will she go anywhere? If she moves what will her direction be? Explain. (c) An astronaut is floating in outer space and wants to move backwards. S he tosses a ball out in front of her. Will she go anywhere? If she moves,  what will her direction be? Explain (d) A college student on roller blades has a carbon dioxide container  strapped to her back. The carbon dioxide jets out behind her under  pressure. Will she go anywhere? If she moves what will her direction be?  Explain. 

Expert Answers

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In A and B, both devices are the classic "Perpetual Motion Machine."  Both are supposed to have a way to theoretically harness an energy to do work.  Unfortunately, for any energy to be transferred the energy source has to be external to the object upon which the work will be done. 

In a) since the fan is on the sailboat, it will not propel the sailboat at all.  If the fan is big enough, it could propel the boat backwards in accordance with Newton's third law, but those fans usually need an internal combustion engine to operate. 

In b) the two magnets are internal to the wagon, so they will not make the wagon move.  The wagon will just sit there, until an outside force acts on it. 

In c, the astronaut will move backwards according to Newton's Third Law "For every action, there is an equal and opposite reaction."  The action of throwing the ball has an equal and opposite reaction propelling the astronaut backwards. 

In d, the student will move forward, the "reaction" to the action of releasing the compressed gas behind her.  This is the principle behind rockets and jets.  How long she will maintain control of that motion on the roller-blades is uncertain, but likely not for long. 

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