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Old Feb 18th 2010, 05:45 AM   #1
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Join Date: Feb 2010
Posts: 1
Unhappy What is the true explanation for this?

hello people ,

I can't understand deduction of thrust force by this way?
This is an extract from my book
http://img693.imageshack.us/img693/3506/

well i have an explanation but not sure about it
thrust exerts a force on the rocket , the rocket answers by a force equal in magnitude and opposite in direction which makes the rocket accelerate upwards

since there is no external force affecting the system (because the system is isolated)
therefore the net force affecting the system is zero (the sum of thrust force and rocket's reaction on thrust = 0)

therefore F=d(mv)/dt=0
where F is the net force acting on the system

by using the product rule we get :

F= mdV/d t+ Vdm/dt=0


Therefore mdV/dt =- Vdm/dt = Fimp
Where the negative sign indicates the direction
so, Right??
If yes then write "right" if no say why?

I have also another question
what does each of mdV/dt and -Vdm/dt indicate???
does mdV/dt indicates the reaction of the rocket upwards and -Vdm/dt indicate the thrust on rocket???

Thanks
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