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A car of mass $1600$ kg is pulling a caravan of mass $800$ kg on a straight horizontal road. The car and the caravan, moving at a constant speed of $25$ m/s, are connected by a light rigid tow bar. The resistances to the motion of the car and caravan are $400$ N and $250$ N. To find the power of the car's engine, won't we only multiply $400$ with $25$? I don't understand why $250$ has to be added to $400$ since $250$ is not the driving force of the car. Please help.

mjqxxxx
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  • This is not a mathematical question. – user Mar 02 '21 at 19:20
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    The total force on the car must be zero, as must the total force on the caravan, since each is moving at constant speed. The forces on the car are the force from the engine (to be determined), the force of air resistance ($400$ N pushing backward), and the tension in the tow bar (pulling backward). The forces on the caravan are the force of air resistance ($250$ N pushing backward) and the same tension in the tow bar (pulling forward). So the tension in the tow bar must be $250$ N. Hence the total force slowing the car is $400+250=650$ N. – mjqxxxx Mar 02 '21 at 19:20

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