# Power consumption of a human running

## Homework Statement

If a 70kg man is running 3 m/s horizontally, how many watts is he consuming?

## Homework Equations

W = N-m/s
N = mass*acceleration

## The Attempt at a Solution

I would assume that in order to get the force in N one would use the gravitational constant of the Earth, but the man is moving in a horizontal line. Regardless, he is technically using work to move 'up' since his center of mass is going slightly up and down. How do I account for this, however?

Thank you.

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NascentOxygen
Staff Emeritus
I would assume that in order to get the force in N one would use the gravitational constant of the Earth, but the man is moving in a horizontal line. Regardless, he is technically using work to move 'up' since his center of mass is going slightly up and down. How do I account for this, however?
Unless you were given typical data for the energy (or force) demanded of a runner I don't think you are going to be able to answer this. Sure, he does lift his CoM during each step, but that energy is returned when he allows it to then fall, so no nett energy expenditure there.

I vaguely recall this same or a similar question from some months back, but I may be imagining things. https://www.physicsforums.com/images/icons/icon6.gif [Broken]

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CWatters
Homework Helper
Gold Member
What he said. You do not have enough information to calculate the power.

If it was 3m/s vertically that would be a different matter. In that case the man is gaining Potential Energy at a known rate so you could calculate his minimum power output.

If the problem is homework or from a book perhaps type it in word for word?

CWatters
Homework Helper
Gold Member
he is technically using work to move 'up' since his center of mass is going slightly up and down.
however what he expends going "up" he gets back going "down". So it's only the "losses" that he incurs in the process that he has to provide. Plus air resistance etc.

In case it helps.. I remember reading that a fit person can produce about 250W for sustained periods. That's the sort of figure I think you have to assume when designing a man powered aircraft. Apparently it's easier to teach a racing cyclist to fly a plane than to train a pilot to produce the required power.