This question is inspired by a sign I saw at a lift, which said something like:
Going up 1 floor or down 2 floors? Take the stairs, it's better for your health and for the environment.
There's no doubt that taking the stairs is better for one's health, but the physicist in me wonders about the latter. An analysis:
- Presumably whoever wrote that sign was thinking about energy usage. By taking the stairs instead of the lift, I save on the electricity required to power the lift.
- However, gravity is a conservative field. The end result in both cases is that I move up one floor, so the amount of work (i.e. energy) required to get me up one floor is independent of the path I take. It should only depend on the initial and final states. Therefore, the electricity saved is compensated for by the fact that I need to eat more food (so my muscles can produce the energy required to move me up one floor).
- If I take the stairs, I do more than $mgh$ of work, because the stairs isn't completely vertical and there's some amount of horizontal work done. Therefore it's actually more energy-intensive overall to take the stairs instead of the lift. (There is also some kinetic energy I have to impart to my body to make it move upwards, but my body also has kinetic energy in the lift, and we assume they are equal.)
- However #3 seems superficial because the final state isn't exactly the same: the lift has also moved up one floor. The mass of the lift is obviously quite large, so to get it up one floor takes more energy.
- But it's exactly because of this that modern lifts use a counterweight that serves as a gravity battery. Whenever the electric motor moves the lift down, some amount of weight is lifted as a counterbalance. When the lift moves up, most of the energy comes from the potential energy stored in the gravity battery; the electric motor doesn't actually do much work. Besides, even if the electric motor does more work, the argument only holds in one direction. If it takes more energy to go up the building (compared to walking), it takes less energy to go down the building (compared to walking).
- If we assume that on average I move both up and down the building, then this difference cancels out, and we're left with the argument in #3.
Conclusion: the sign is wrong. It's (slightly) worse for the environment to take the stairs instead of the lift.
I'm wondering if this analysis is good or if I missed something.