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I was thinking... We could calculate the power of a runner by using the equation $P = fv$, supposing his velocity constant. But so raise me a question: What makes possible to run is the friction, so there is one force pointing acting on him in forward direction since he is pushing the floor back. Now, if he is at constant velocity, the net force should be zero, the question is so what is the other force pointing behind the person?

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  • $\begingroup$ if you know about a force in action then it is sure that it can't move with a constant velocity . $\endgroup$
    – Ankit
    Oct 25 '20 at 18:29
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It is only possible to run due to the friction with the ground. But during the cycle of each foot, the friction force changes from backwards (when the runner touch the ground with one foot) to forwards (when the other foot is almost touching ahead for another cycle). If it is integrated over the cycle (disregarding the air resistance) the result must be zero, otherwise the runner would be accelerating.

It can also be seen in a treadmill without motor, and with a good bearing lubrication. After an initial acceleration, the system is rotating at a constant rate, showing that the runner is not permorming any net torque on it.

The force responsible for the power wasted is in the upwards direction. The body CM starts each cycle a little lower than average, reach its maximum height and lowers again. That is why it requires more effort to run either a inclined real lane, or a inclined tread mill. The portion of the cycle when the runner is going up increases in this case.

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