# How many slices of pizza must you eat to walk for 1.5 hr at a speed of 5.0 km/hr

## Homework Statement

Assuming a typical efficiency for energy use by the body, how many slices of pizza must you eat to walk for 1.5 hr at a speed of 5.0 km/hr? (Assume your mass is 68kg.) We know that
the energy content in ONE slice of pizza is 1260 kJ and the metabolic power of this activity is 380 W.

## Homework Equations

Delta t (change in time) = Delta E (energy)/P(power)
Delta x = v(speed)*delta t (time)

## The Attempt at a Solution

I started by calculating delta x, which is 7.5 km total.
I then tried to solve for the change in Energy by taking the power 380 and multiplying it by the time 5400 s (1.5 hr). Since ONE slice is 1260 kJ I thought that I would only need 2 slices but this answer is incorrect. I suppose I would need to first find how much energy it takes to walk at this pace for that amount of time and then compare it to the amount of energy in once slice of pizza. This question is for an extra point so it is not necessary that I do it, but I would like help so that I may figure it out for myself. Any help would be appreciated.

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Borek
Mentor
Your initial approach seems to be correct (that is ignoring delta x, no idea what you mean by that). Perhaps they want you to give more precise answer (ie not whole slices).

And you know, this is hardly advanced physics...

You have to remember that the human body has 25% efficiency. Therefore divide 1260 by 25% which will give you the amount of energy your body will use from a slice of pizza. Then take whatever answer you get for amount of needed energy (W x T in sec.) and divide that by the amount of energy your body actually uses from a slice of pizza.

You have to remember that the human body has 25% efficiency. Therefore divide 1260 by 25% which will give you the amount of energy your body will use from a slice of pizza. Then take whatever answer you get for amount of needed energy (W x T in sec.) and divide that by the amount of energy your body actually uses from a slice of pizza.
I think the efficiency is already factored in the 380W needed for a 68kg person to move at 5 km/hr. That amount of net power/weight will get you close to world record marathon speed.