# Physics

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While running, a person dissipates about 0.6 J of mechanical energy per step per kilogram of body mass. If a 60-kg person runs with a power of 70 Watts during a race, how fast is the person running? Assume a running step is 1.5 m long.

• Physics -

70 J/s = (V/1.5)step/s*(0.6 J/step kg)*60 kg

Solve for V in meters/s
V = 70*1.5/(0.6*60) = 2.9 m/s

That is a pretty slow speed for a runner. It's roughly 9 minutes miles... more like jogging.

• Physics -

Thank you!

• Physics -

A person on a diet might lose 1.8 kg per week. Express the mass loss rate in milligrams per second, as if the dieter could sense the second-by-second loss.

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