Two long barges are moving in the same direction in still water, one with a speed of 10km/h and the other with a speed of 20km/h. While they are passing each other, coal is shoveled from the slower to the faster one at a rate of 1000 kh/min. How much additional force must be prvided by the driving engines of (a) the faster barge and (b) the slower barge if neither is to change speed? (There is a picture to this but I think the description of the problem is well enough)(adsbygoogle = window.adsbygoogle || []).push({});

Assume that the shoveling is always perfectly sideways and that the frictional forces between barges and the water do not depend on the mass of the barges.

This is a problem from Fundamentals of Physics 8/e Ch9 #79 on the back of the chapter.

(BTW this is not homework, just some randomly popped up question in my head)

The way I can solve this problem, which corresponds to the answer on the back of the book is to find the change of momentum of the coal when they are being transfered from one to the other.

However, the problem that I'm having with is that I cannot seem to find a way to use F=v*dm/dt + m*dv/dt to solve this particular problem?

Is it because F = dp/dt applies only to constant mass or can anyone provide a title for a book that has problems using this particular formula? ( Seems to me that there isn't any such problems in this whole book)

The answers are a) 46 N b) 0 since the engine does not need to provide any force

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# Homework Help: Force = Change of Momentum?

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