In the figure, two long barges are moving in the same direction in still water,

Question

In the figure, two long barges are moving in the same direction in still water, one with a speed of 5.0 km/h and the other with a speed of 20 km/h. While they are passing each other, coal is shoveled from the slower to the faster one at a rate of 810 kg/min. How much additional force must be provided by the driving engines of (a) the faster barge and (b) the slower barge if neither is to change speed? Assume that the shoveling is always perfectly sideways and that the frictional forces between the barges and the water do not depend on the mass of the barges.

Explanation / Answer

The fast one becomes 810 kg heavier in one minute. So the momentum to preserve is 5km/h * 810 kg in one minute.F = (5 km.810kg/h)/(1/60)h =243000km.kg/h^2 = 675 kg.m/s^2 = 675N

For the slow one, it's twice that, which is 92,6 in the other direction so = - 1350N

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