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Force is change of rate of momentum.
Momentum is p=mv
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To work out the change of rate of momentum (and therefore the force) you can do:
1)
∆p/∆T (change in momentum over change in time) This is the same as change RATE of momentum
or
2)
m∆V/∆T (because p=mv and mass doesn't change)
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If mass is changing (like a rocket ejecting fuel and becoming lighter) but velocity stays the same, the force will still change.
So we get the equation:
3)
V∆m/∆T (velocity doesn't change)
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But this is only an average.
The momentum might increase quickly at the beginning of the time frame (∆T) and slower at the end.
This would mean the force is larger at the start and smaller at the end.
So the force is only an average.
(To make the force more accurate you make the ∆T smaller)
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Not sure if this answers you question but I hope it helps