Energy is conventionally measured in Calories as well as in joules. One Calorie in nutrition is one kilocalorie, defined as 1 kcal = 4186 J. Metabolizing 1 g of fat can release 9.00 kcal. A student decides to try to lose weight by exercising. She plans to run up and down the stairs in a football stadium as fast as she can and as many times as necessary. Is this in itself a practical way to lose weight? To evaluate the program, suppose she runs up a flight of 80 steps, each 0.150 m high, in 65 s. For simplicity, ignore the energy she uses in coming down (which is small). Assume that a typical efficiency for human muscles is 20.0%. Therefore when your body converts 100 J from metabolizing fat, 20 J goes into doing mechanical work (here, climbing stairs). The remainder goes into extra internal energy. Assume that the student's mass is 50.0 kg. How many times must she run the flight of stairs to lose 1 lb of fat(adsbygoogle = window.adsbygoogle || []).push({});

I got an answer for this question, but the number is too large, so I thought I may have gotten something wrong in my calculation:

okie, so if 1 g of fat can release 9.00 kcal, then 1lb of fat can release 17124545 J.

0.5mv^2 + W = mgh

However, to get the initial velocity (v):

Vf^2 = Vi^2 + 2ax

0 = Vi^2 + 2 * -9.8 * (0.15 * 80)

Vi = 15.3m/s

Plug it back in:

0.5 * 50 * 15.3^2 + W = 50 * 9.8 * (0.15 * 80)

W = 27.8 J

However, since human is 20% efficiency...

27.8 J * .2 = 5.6J

So running 80 set of stairs will let you lose 5.6 J of fat. So to lose 1lb of fat:

17124545J/5.6J = 3057954

To lose 1 lb of fat, you need to run the 80 stairs 3057954 times! For some reason, I think this value is too large. Anyone caught a mistake on my calculation?

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: An energy problem

**Physics Forums | Science Articles, Homework Help, Discussion**