The question it self is bad cause most people (as yourself’s) don’t understand energy very well!
If your talking about Human energy (calories and that stuff) then even a child knows it takes more enery to go against gravity than with it!!!
If your talking about real enery then you must realize IT IS EXACLY THE SAME! All you do is change potential to Kenetic (neglecting friction due to the small speeds).
Look up conservative systems in a scientific Encylopedia and you’ll get some idea about what is Energy.
As for who ever wrote the question: maybe you should think about the answer first and ask more intellegent questions!
Eddie just summed up everything I have said. And you don’t have stairmasters going downhill because there is a negligible aerobic benefit.(Your muscles do less work)
Ok, I hate typing long responses, but I am tired of this topic. So here goes.
Muscle contractions are not all or nothing responses. You only contract as many sarcomeres as you need. This is why you do not slam a can of coke into your face when you lift it. This is also why you cannot hold a weight suspended forever. As your muscles fatigue, more and more sarcomeres are recruited, until there are none available and failure results. Assuming equal weights(your body, in this case) any concentric contraction requires more sarcomeres to be involved than the corresponding eccentric contraction.
Ok point 2-
Energy derived from the food(sugars and starches) that we eat is stored as energy in the form of adenosine triphosphate (aka ATP). As you will remember from my earlier lecture, ATP is the chemical which binds to each myosin head as it release from its binding site during a contraction. Now if we just put 2 and 2 together, we understand that-
any concentric muscle contraction requires more sarcomeres working than any eccentric contraction(assuming equal weight). The more sarcomers you have contracting, the more ATP is needed to bind with the myosin heads as they release. ATP is the chemical in which the body stores its energy. It is the chemical which allows the myosin heads to “reload” and contract again. Since you need more ATP to produce a concentric contraction(assuming equal weight), your body is burning more “energy”.
Eddie – roksez – your “explanation” has the slight practical disadvantage that it gives the same answer for all experiments, practical or gedanken, that do not involve divine intervention or the use of magic. Of course energy is conserved! But when potential energy is entered into the equation, you can say equally well that an ant sneezing uses the same energy as a supernova.
You’re arguing like a doctor who answers the question: “Will my wife live or die?” with “Yes.” Technically true, but useless and rude.
John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams
Strainger: Hiking is not the same thing as stairs, there’s a lot of friction when you walk down an incline but not much when go down stairs. Also read there is the other thing which John Larrigan said:
JohnLarrigan: Actually, that’s a good point. I didn’t want to bring it in before because it would confuse things but since you brought it up… It does take more energy cause you have to make yourself move forward when you go up but once you make that first initial push forward at the bottom of the stairs you pretty much keep the momentum. When you walk up the stairs you sort of bend forward and push almost straight down. I’m sure you lose some of the forward momentum from friction and stuff but I don’t think it’s a major effect. (If you have gain .5m/sec horizontal speed each step it takes .125 joules/kilo of body mass whereas going up one step takes 2 joules/kilo.)
roksez: Out of your 2 points how do you get that it takes more energy for concentric than eccentric? I’m not saying it’s not true but you just made a statement, not an argument. You have no proof for that. Either way as I said before we’re not counting any friction so that point is irrelevant.
What is also irrelevant is the fact that your body uses ATP for energy. I took bio, I know what ATP is. What are you trying to do? Impress me by pointing out an obvious and COMPLTELY irrelevant fact? It wouldn’t bloody well matter if your body used goat sperm for energy, it would use just as much goat sperm going up as down if it weren’t for friction.
John Kennedy: I think what Eddie is trying to say is that because energy is conserved then all the extra energy you used going up must come back out as friction when you go back down. The whole point is when you go downstairs you spend a whole lot of energy as friction dragging your hands on the rails and so on and that takes the load off your muscles.
A prime exmple of why the original poster needs to post a link. Cecil said every bit of this with a lot less and simpler words.None of that extensor vs. contractor,chemical processes etc has any thing to do with it. In physics you allways assume a perfect enviroment,africtionless,gravityless,vaccuum, then start adding conditions. Sliding down a bannister causes friction which results in heat which is energy.Same amount of energy as heat as it took to climb? Probably in that perfect place with friction and gravity added. A downstairsmaster? What a concept,I’ll be in the basement developing that,if Igot the energy to go downthere.(My brother’s down there now anyway cussin at the government or something). Any way can I ask you physiology people the question this way? "Do I use different muscles to go upstairs or down? If so, is one set more efficient than the other? If not,are they more efficient one way or the other? In other words, do less efficient muscles take more “energy” to operate? Does my body have to "work’ harder? Note the quotes physics people. It takes just as much energy to conteract gravity when raising something as it does to resist it when lowering it at the same speed. Get a rope, pulley,scale, and weight and check it out.Now if i tie the rope off with weight suspeded no energy is being created or used, it is all stored(kinetic) energy. But if I hold the rope in my hand,then for sure my body is expending energy.Ty this stand at the top of the stairs like this and LOOKOUT! OOF BLUHHH WHAP CURSE OOOF YOU OOF NEWTON! OUCH ARGHH KIN AHHHH ETIC OOF POW> BANG
That enough spaces Dex?
Signitorily yours, Mr John
" Pardon me while I have a strange interlude."-Marx
Wow, this whole thing turned into an argument with a moron? Dangit, I miss all the good stuff.
Do a search on deja news for “Konrad stairs” under sci.physics. Most people agreed with me.
Also the 2 physics Ph.D.'s I asked said I was right, and I think they’re a lot more likely to be right than some moderator on a message board.
This reminds of the old claim that pops up now and then: “Mathematically, it is impossible for a bumblebee to fly.”
Save your breath and no, I ain’t interested in your cipherin’. Fact is, I seen the sumbitches fly. 'Nuff sed.
Hmmmmmmm pretty bad when even the moderators can’t approach an issue with rational discussion.
If the bunch of you would stop talking apples and oranges, you might reach an agreement.
First of all, the question originally posed was: “I have a dispute with my
significant other. She claims that going down stairs takes more energy
than going up stairs. At first glance, anyone can see that this is
clearly wrong … or is it? She says that when you go down stairs, you
have to absorb the energy from gravity pulling you down, and that takes
energy on your behalf. I give her the argument that I get much more tired
going up 50 flights of stairs than down!”
To answer the question: Does it take more energy to climb or descend a flight of stairs? one needs to define the terms of the question.
Konrad is defining the term ‘energy’ using its meaning in Physics. As was cogently noted in Resnick and Halliday (“Physics, Part I” Robert Resnick and David Halliday 3d ed. 1977) at pg 119, “A person holding a heavy weight at rest in the air may say that he is doing hard work - and he may work hard in the physiological sense - but from the point of view of physics we say that he is not doing any work. … In many scientific fields words are borrowed from our everyday language and are used to name a very specific concept.” If one says ‘energy’ and means ‘vigorous exertion of power’ (see Mirriam-Webster), one can’t argue this issue with someone who means ‘the capacity for doing work.’
Having said that, let us look at a force diagram (in our mind, since I can’t draw one here). First, let us simplify the diagram to eliminate the horizontal component, since the horizontal component of travelling up and down stairs is the same each way (same distance, same mass, same work). (By the way, this is why talking about friction makes no sense in this question… it affects horizontal motion only) Now, we have to agree HOW to draw the diagram. To do that, we have to agree upon the parameters. Konrad is assuming that the mass in question (the person) starts at zero velocity and ENDS at zero velocity, in the vertical direction. The equivalent would be a ball, dropped from a hight onto a spring that slows the ball to 0 velocity vertically. The force diagram is simple. There is downward force (gravity) accelerating the mass in a negative y direction. There is an upwards force (the spring), accelerating the mass in a positive y direction. Going up the stairs, the same things are true. Gravity pulls down, but the spring pushes up. The ‘energy’ involved is the same, because the same mass, distance and net acceleration are involved. (If you don’t accept this, I suggest you review the basic occilating spring problem). Both Konrad, AND the spouse in the original question, assume that the positive y force is provided by the human.
The Anti-Konrad forces are making different assumptions, or defining ‘energy’ differently. They correctly note that to go up the stairs, the body must do work, that is, exert force to propel the body upward, but that the body need do nothing to propel it downward, the force of gravity accomplishing that task. Since the body isn’t moving in a positive y direction when descending, then it is expending less ‘energy’ (meaning something closer to ‘vigorous exertion of power’). The definition answers the question.
However, I must point out that the original question SHOULD be answered in terms of physics, since the spouse was making a physics argument. Under that circumstance, Konrad is quite right in his assertions, despite the rather ad hominem responses to the contrary.
As for the answer from SDSTAFF Dex, no offense, but the Staff should really try to answer a question with some actual thought, rather than making an offhand response based on potentially incorrect assumptions. The answer TOTALLY ignores the force needed to accelerate the object positively after gravity has accelerated it negatively, and that is NOT the same as the force needed to just hold us upright.
Sheesh, some of the answers we get…
Allow me to briefly dispose of the ‘how your muscles feel’ responses to this question. Crawl on the ground 100 yards pulling yourself with your arms, then walk the same distance. What makes you more tired? Which used more energy? (Answer, crawling, and neither).
Sometimes, the issue isn’t how you feel, but what you used…
OK, once more from the top.
IF the question had been whether it takes more energy to lift an object in the air or to lower it gently the same distance, then the argument from physics says that it takes the same amount of energy.
Please note: if the object was not being lowered gently, but was being lowerd most of the way and then dropped… then it would take less energy (exerted by the person) to lower/drop the object than to lift it.
IF the question was about whether it takes more energy (exerted by the human) to push an object (like a ball) to roll down the stairs, vs throwing it up the stairs, I think we agree that it takes less energy to let the ball roll down the stairs.
Neither of those were the question.
The question was about a human walking down and up stairs. The energy involved in walking is different from the energy involved in lifting an inert body.
I have used the rolling ball as the example, but I probably should have used a person “jumping.” If I stand on the top stair and take a little jump, I go down one stair. This takes less energy (I contend) than if I tried to jump (or walk) UP one stair.
The physical process of walking down stairs involves a little “lifting” and a little “jumping” – letting gravity set your foot down. To that extent, gravity is helping you go down, but does not help you go up.
Does that resolve the seemingly irresolvable differences?
On a further note, Konrad, you can take the gratuitous insults to the BBQ Pit. This is not a question that is resolved by majority vote, not even if two Ph.D.'s vote with you. The author of the Mailbag item has a Ph.D., and he sought (and got) agreement with a physics Ph.D. prior to posting the article. So we’re even. I don’t think that your attempt at one-upmanship is productive.
I have edited the first posting on this topic to include a link to the Mailbag item.
I see in rereading some of the past posts that we can probably make an additional Mailbag item out of some of the different interpretations (which, admittedly, did not occur to the author of the original column at the time.)
Absolutely correct, if all you want to do is consider the effort needed to start down the stair. Gravity will force you down the stair without any effort on your part (the effort to move horizontally being the same regardless of vertical direction involved).
HOWEVER, we are not just starting down the stairs, we are stopping when we get there. We can’t consider only the effort needed to accelerate vertically down, we have to consider the effort to DECcelerate vertically down (that is, accelerate vertically up). THAT effort, absent some springiness to the floor, is also provided by our body. Specifically, your foot, ankle, leg, knee, hip, back, etc. combine to slow you down, so that you don’t continue on through the floor. If you don’t manage to do it in a co-ordinated fashion, you end up like I have on more than on occaision, sprawled on the floor having absorbed the energy of the collision in a VERY inelastic fashion.
Throwing the ball up would take more energy than letting it fall. But the correct question is, how much energy would it take to catch it? Sending a rocket up would take a lot of energy, but the same energy is needed to brake a falling rocket so that it doesn’t crash.
If you don’t believe the energy involved in going down the stairs, try falling onto the floor from one story high. Trust me, your body will feel the energy!
But do we think of that as ‘effort’? Likely not… again, a question of definitions.
Well, if it comes to that, consider the motion of walking up a stair. You raise your foot HIGHER than the level of the stair, and then lower your foot onto the stair. (Similarly, in walking down stairs, you first lift your foot a slight height in order to move it off the step.) So the effect of “stopping” gravity from pulling you down is the same, whether going up or down.
In any case, we now have some different takes on this that were not considered in the original Mailbag post –
- biology (the muscles and bone used in going up vs going down)
- pure physics (motion of an inert body) and the technical definition of “energy”
- further insights into the definition of “energy” in the common sense of effort expended
Anyone wanna tackle number of calories used, or is that the same as (3)?
Oh, please. Now we reach the absurd. The issue would be the same if you stepped higher then let a foot down, or if you were precise in your mechanics.
The original question from the couple couldn’t be answered is my point. The wife, like Konrad, was talking physical energy. The husband, like Dex and many others, was talking effort, a subjective physiological feeling. The energy expended to stop the step down is not felt the same as the energy expended to lift the body up. The couple, as are the teeming millions, were talking two different things, which the original answer posted should have noted.
As for calories, again it depends on what you mean. Do you mean total calories expended by the human, or calories of energy involved in the physical process of raising a mass up the stairs or stopping it at the bottom of the stairs?
Example: run 100 yards, then walk 100 yards. The energy in moving the mass is the same, if there is constant velocity involved. BUT, the body expends more calories to run, because the machine involved is less efficient at higher speeds. The analogy would be the same for a car, which will get different gas milage depending on speed. So the calories expended climbing versus descending may change depending on which the body is better at doing.
Anyone want to run some trial climbs and measure? <eg>
I mean calories expended by the human… that’s the only topic under consideration.
Restating the question: the escalator going down is broken, but the escalator going up works. Does it take more energy to go down or up? Sorry, but I believe the plain reading of the question (like the original question in this thread) has to do with what causes the human being to expend more effort, to burn more calories, to use up their energy (note: energy in this sense has nothing to do with physics because I’m claiming it to be USED UP.)
The other interpretations are interesting, no question, but tangential.
I’d just like to point out that I specifically stated my interpretation of the question when I posted. I said no friction, no special forces from the human body. So if the disagreement comes from a matter of definition then it’s a problem of people not reading my original post. Like I consider moving your foot up above the stair and then putting it down a “special force from the human body”.
And yes friction does make a difference in the y component. If you drag your hand on the handrail as you go down you are using friction to diffuse the energy of going down. That’s one of the reasons you don’t get as tired going down. So the problem is not that I’m using a different definition of energy. If we take out friction both defintion must be the same.
CKDext: It’s you who’s been rude and patronizing all the while so don’t get huffy because I went and got the opinions of some experts in the field.
Okay folks, think of it this way.
I’m standing at the top of a short staircase. Instead of walking down the stairs, I step sideways off the staircase and fall straight to the ground. Before landing, I lock my knees, so that I hit the ground with a loud bang and maybe a slight bounce. (We’ll assume that I don’t hurt myself doing this.)
How much energy have I expended in vertical motion? Zero! I was accelerated in the negative direction by gravity, then accelerated in the positive direction by the normal force exerted on me by the floor. On impact, my kinetic energy was converted to sound and heat, and possibly some damage to the floorboards.
Now I teleport back to the top of the steps. This time instead of stepping off and falling the whole way down, I step off each stair, and fall to the next one in line. I still lock my knees before impact. For each step there is a bang! as I am accelerated to a stop.
Once again, I have expended zero energy in vertical motion. I provide horizontal motion; gravity makes me move down; the normal force makes me stop.
Teleport back to the top again. This time I walk down the stairs normally. I exert some energy to slow myself down as I descend, and some amount of energy is still “bled off” into sound and heat.
The amount of energy I expend in vertical motion while walking downstairs is equal to my potential energy at the top of the steps, minus that absorbed by frictional effects here.
Now I turn around and walk up the steps. Each time my foot hits a stair, some small amount of energy is lost to sound and heat.
The total energy I expend in vertical motion while walking upstairs is potential energy plus the energy I must expend to overcome friction. (The potential energy is obviously the same number as in the above situation, since my location is the same.)
Let’s see those numbers again:
Energy to walk down = Potential - Friction
Energy to walk up = Potential + Friction
If you are ignoring friction, as Konrad did, then the required energy in the two cases is identical.
However, this question was asked about the Real World ™. In the Real World, friction always exists, and is always positive. Therefore, in the Real World, it will always take more energy to walk upstairs than to walk downstairs.
Sorry for the long post, but I hope it’s now clear what everyone was arguing about.
Of course I don’t fit in; I’m part of a better puzzle.