To all the Thermodynamics Enthusiasts..

[Takeno]

Hello all,

I am studying electrical engineering and barely remember much from my Physics 1 course from my freshman year in college. The book for my class still hasnt come in the mail and I just cannot figure out how to work out this problem. If anyone can help, I'd really appreciate it.

2. A person with a mass of 60 kg and an energetic eciency of 30% decides to burn all the calories consumed from a can of
soda (140 food calories) by climbing the stairs of a building. If the average height of a
ight of stairs is 15 m, calculate the
number of
ights they would need to climb, and the average power output in W if it takes 30 s to climb a
ight of stairs.
[1 food calorie is 1 kcal.]

[Byrthnoth]

15m*(60kg*9.8m/s^2) = W = 8820 kg*m^2/s^2 (Joules) per flight of stairs
W/t = P = 294 kg*m^2/s^3 (Watts) on average when climbing

Now, where you put the inefficiency is going to depend basically on what you've heard in class, I'd bet. I would personally divide the work term by it and say they're doing more work, but you could also multiply the kcal input by it and account for it that way. This is only important because it will affect the "average power" calculation. I'd give it 80:20 odds that you're supposed to factor it into the work term so it shows up in your Power calculation. This would be one part of the problem that you'd benefit from having a text book on.

15m*(60kg*9.8m/s^2) = W# = 29400 kg*m^2/s^2 (Joules) per flight of stairs with inefficiency factored in
W#/t = P = 980 kg*m^2/s^3 (Watts) on average when climbing with inefficiency factored in

1kcal = 4184 Joules

140*4184/29400 = 19.92... they probably used a slightly different conversion factor for joules (4200). The answer is supposed to be 20 flights of stairs.

PS. I hated PChem/Thermo, but this isn't really either of them. This is kinda like an AP Physics question. It's just unit conversion.

[Takeno]

Thanks very much!

This is the first homework assignment of the semester so that is why it seems like a review of Physics. I just want to get over this thermodynamics class so I can continue doing some more electrical work.