Force and Momentum The net external force acting on an object can be evaluated as the rate of change of momentum. This turns out to be a more fundamental way of stating the force than the use of Newton's second law. Using Newton's second law and momentum:
In the language of calculus, we say that the force is defined as the derivative of the momentum. The process above involves the product rule for derivatives. 
Index Limits on Newton's 2nd law  

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Rocket PropulsionThe thrust of a rocket can be modeled from a generalization of Newton's 2nd Law to include a variable mass:

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Thrust of a RocketRocket thrust results from the high speed ejection of material and does not require any medium to "push against". Conservation of momentum dictates that if material is ejected backward, the forward momentum of the remaining rocket must increase since an isolated system cannot change its net momentum.

Index Rocket propulsion  

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Thrust of a Rocket

Index Rocket propulsion  

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Thrust of a Rocket

Index Rocket propulsion  

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Rocket Thrust DiscussionDeveloping the expression for rocket thrust involves the application of conservation of momentum in an accelerating reference frame. It can be developed in an approximate fashion using finite differences and algebra. This requires neglecting terms like . When the appropriate limits are taken to get an instantaneous expression, these approximations disappear since the quantities dM and dt approach zero.

Index Rocket propulsion  

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