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Question:
Where would the balance point be for a water rocket using a 2L bottle half filled with water and about 50 lbs air pressure. Would the force be concentrated near the top of the bottle as the bottle pushes against the escaping water? bottle rocket force thanks!



Replies:
Robert,

I am not quite sure what you mean by the balance point. Could you explain As far as where the force is greatest, neglecting the gravitational forces (due to the weight of the water) the force results from the pressure of the air in the bottle. Since the contents of the bottle (air and water) are fluids the pressure they exert on the bottle walls is the same everywhere and since force is the product of area and pressure the force is the same everywhere in the bottle. If you are thinking of moving the bottle by letting its contents out you will get better results by letting the water out first. In this way .... The momentum of the system is conserved. The fast moving water leaving the bottle imparts motion to the bottle. Since the bottle is losing water it is getting lighter. The (nearly) empty bottle must move much faster than the last of the water in order to conserve m momentum.
Hope this is useful.

gregory r bradburn



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