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A Question for You Physics Experts


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  • #1939075
    Diplomatic Mike
    Member

    @mikefaedundee

    Locale: Under a bush in Scotland

    Same vehicle. :-)

    #1939098
    Nelson Sherry
    Member

    @nsherry61

    Locale: Mid-Willamette Valley

    Shifting from a three to a four hour run isn't going to change metabolism substantially, since either way, you aren't going to be finishing the run if you work much over your lactate threshold. Running 25% faster probably will play a big roll in how efficiently your digestive system works, so you may be running out of energy because you are using it up faster than your body can resupply the muscles. But that isn't using more energy, it is just using up a more limited energy supply.

    Also, we have a running gate and a walking gate that is most energy efficient given our personal training and biomechanics. If we are running or walking either slower or faster than our most efficient gate, we are using more energy. Hiking with a hiker slower than me is exhausting, as is hiking with someone that walks or runs much faster.

    Finally, more time standing is more energy used. So, if you spend four hours running vs. three hours running and one hour laying down, the overall energy usage would be closer than comparing the energy usage of only three vs. four hours of running time.

    Okay, one more point . . . I don't think this is really an appropriate question for a physics expert, since physics actually has relatively little to do with it. It seems more appropriate for a physiologist. :)

    #1939099
    Bob Gross
    BPL Member

    @b-g-2-2

    Locale: Silicon Valley

    "Also, we have a running gate and a walking gate"

    No. How about gait?

    –B.G.–

    #1939195
    Jennifer Mitol
    Spectator

    @jenmitol

    Locale: In my dreams....

    Haha. Thanks Bob…..

    It is an interesting question for physics nerds (of which I am one…), but I agree that if you want a real answer it is actually a question for biomechanists and physiologists. What the OP asked boils down to running economy, fitness level, efficiency, etc.

    Ever had a day when even though everything seems like it should be fine, you just don't have the legs? We call it an "off" day and they are quite common across all athletic pursuits, regardless of level. It is also a common maxim that it is MUCH harder to run a 6 hour marathon than a sub-three, but that 6-hour "runner" is incapable of actually performing at the level required to run a sub-three. He doesn't have the mechanics, the VO2 max, the gait, the stride, the percentage of fast vs slow twitch muscle fibers, etc. Yet the amount of damage to his body, the energy expended, etc is FAR greater for the poor slow poke the back (again…count me in!) than the elite guy at the front.

    If we were to take only mechanics into consideration, none of this would be true. It works great for all those calculations of energy and work and mass and time etc, but unfortunately our bodies have a heck of a lot more going on than simple Newtonian physics. Well, actually it really is all just Newtonian physics…but a whole mess of it!!!!

    Here is an article from the NY Times that is a good summary: http://www.nytimes.com/2007/10/11/fashion/11Best.html?pagewanted=all&_r=0
    It does a good job of explaining all the variables to running efficiency: fitness, running economy etc.

    This is a huge area of research right now for those of us who work with athletes.

    #1939212
    Jerry Adams
    BPL Member

    @retiredjerry

    Locale: Oregon and Washington

    "I don't think this is really an appropriate question for a physics expert, since physics actually has relatively little to do with it. It seems more appropriate for a physiologist"

    The physics experts would be better at asking the question whether you'de get wetter, if it was raining, running faster or slower

    #1939220
    Greg Mihalik
    Spectator

    @greg23

    Locale: Colorado

    "The physics experts would be better at asking the question whether you'd get wetter, if it was raining, running faster or slower."

    Physicist Are good at asking questions…

    And providing answers –

    You'll get "less wet" by moving faster.

    #1939227
    Franco Darioli
    Spectator

    @franco

    Locale: Gauche, CU.

    You get less wet running faster.

    well this is the typical area where I disagree regardless of how many experts come on board.
    Here is why.
    Go out when it rains really hard just wearing your standard running stuff.
    After say 30 minutes max you will be completely wet regardless if you are walking or running.
    Once you are fully soaked you can't get wetter than that…

    So , it depends….

    anyway going back to calories here is a bit from this article :

    http://www.runnersworld.com/weight-loss/how-many-calories-are-you-really-burning-0?page=single

    I was still gathering my resources for a retort when a new article crossed my desk, and changed my cosmos. In "Energy Expenditure of Walking and Running," published last December in Medicine & Science in Sports & Exercise, a group of Syracuse University researchers measured the actual calorie burn of 12 men and 12 women while running and walking 1,600 meters (roughly a mile) on a treadmill. Result: The men burned an average of 124 calories while running, and just 88 while walking; the women burned 105 and 74. (The men burned more than the women because they weighed more.)

    than goes on to talk about Net Calorie Burn :

    Thanks to the Syracuse researchers, we now know the relative NCB of running a mile in 9:30 versus walking the same mile in 19:00. Their male subjects burned 105 calories running, 52 walking; the women, 91 and 43. That is, running burns twice as many net calories per mile as walking. And since you can run two miles in the time it takes to walk one mile, running burns four times as many net calories per hour as walking.

    #1939242
    Art …
    BPL Member

    @asandh

    I think there is room for both physicists and physiologists in my original question.

    As the OP, I posted it to physicists first because I wanted to make sure what the answer was "holding everything but time constant".

    this answer seems to be that doing the run in 3 or 4 hours would use the same energy.

    Now its up to the physiologists to bring in all the inefficiencies that prove in a real world situation that one or the other is more efficient (most likely the slower time).
    some points have already been mentioned.
    but there are so many variables here …

    #1939261
    Bill Segraves
    BPL Member

    @sbill9000-2

    As Jen and others who are thinking that PHYS in this case has to include both physics and physiology, this is all about efficiency.  A few good freely available references are at http://www.ncbi.nlm.nih.gov/pubmed/17766303
    and http://www.clinicalgaitanalysis.com/history/Ralston_IZAP.pdf

    N.b., slower is more efficient only to a point. There's an optimal gait and speed even on level ground, but think, too, about going downhill. If you go very, very slowly, holding back on every step down, you're going to burn a ton of extra energy (think weight-lifting or pushups done in slow mo).

    Cheers,

    Bill

    #1939287
    Stuart R
    BPL Member

    @scunnered

    Locale: Scotland

    Note that the calorie calculator at the bottom of the runnersworld link above has

    Total calorie burn per mile (running) = 0.75 x weight (lbs)

    This is almost exactly the same as the equation I posted on the previous page

    Total calorie burn per km (running) = weight in kg

    #1939301
    Ben C
    BPL Member

    @alexdrewreed

    Locale: Kentucky

    I'm with Jen on this. An engineer /physics person is going to tell you the human body has done zero net work on this journey. You really need a human body expert like a physiologist who can tell you the differences in efficiencies of the human body at different speeds, on average.

    #1939312
    Greg Mihalik
    Spectator

    @greg23

    Locale: Colorado

    "An engineer/physics person is going to tell you the human body has done zero net work on this journey."

    NO engineer/physicist will tell you the Work walking around a closed loop is zero. As posted above, that is only for the specialized cases involving conservative force fields in scenarios that a hiker will not encounter.

    #1940057
    Nick Gatel
    BPL Member

    @ngatel

    Locale: Southern California

    I am no expert.

    But if an elite runner ran a 10K in 40 minutes, then later in the day ran a 28 minute 10K he would burn a lot more energy during the 2nd run.

    #1940083
    Jeremy and Angela
    BPL Member

    @requiem

    Locale: Northern California

    As humans don't have much in the way of regenerative braking, I've set out the following scenario for two speeds. Please let me know if I've made any errors in the calculations!

    The setting: a 75kg person, traveling 100m. Assume 2 seconds are used accelerating to either 1.5m/s or 3m/s.

    F=ma, W=Fd, and d=0.5at^2 so:

    F1 = 75kg * (1.5m/2s^2);
    F1 = 56.25N
    d1 = 0.5 * (1.5m/2s^2) * (2s)^2
    d1 = 1.5m (He will travel 1.5m in this time.)
    W1 = 56.25N * 1.5m
    W1 = 84.375 J

    F2 = 75kg * (3m/2s^2);
    F2 = 112.5N
    d2 = 0.5 * (3m/2s^2) * (2s)^2
    d2 = 3m (He will travel 3m in this time.)
    W2 = 112.5N * 3m
    W2 = 337.5 J

    Disregarding friction, we can assume that person then coasts at this speed until near arrival at destination, at which point he digs his heels into the ground to stop.

    Now, the equation for net work is:
    Wnet = 0.5mvfinal^2 – 0.5mvinitial^2

    Obviously starting, traveling, and stopping would give a net work value of 0, but I think one must consider the energy flow. The initial acceleration has vinitial = 0m/s and vfinal = 1.5 or 3m/s, and represents chemical energy being converted to kinetic energy. The second "acceleration" has vinitial = 1.5 or 3m/s and vfinal = 0m/s, and represents kinetic energy being converted to heat (friction braking). Thus, the caloric cost is mirrored by W1 and W2 above, i.e. roughly quadruple for the faster speed. So, what am I missing?

    Edit: added final W1 and W2 values, changed "double" to "quadruple".

    #1940084
    Nick Gatel
    BPL Member

    @ngatel

    Locale: Southern California

    "So, what am I missing?"

    Uh, plain English?

    ;)

    #1940087
    Tony Ronco
    BPL Member

    @tr-browsing

    Here is an attempt to address Art's question with a small attempt at brevity :-)

    First Some Cerebral Background:
    The human gait cycle has multiple parasitic losses.
    Two of the primary losses are:
    1.) vertical losses due to a person's center of mass moving a small amount up & down during the gait cycle and
    2.) horizontal losses due to transient leg accelerations / decelerations to maintain ambulation at an average velocity.
    (There are plenty of other losses such as the collision of the heel strike, etc., etc.,) but these two will suffice for now.

    So, just considering these two factors; the faster the gait, the greater (frequency of) the acceleration/decelerations (vertical and horizontal) will be needed.
    … That translates to greater force being needed … and that translates to greater energy being needed (to complete the distance)

    Now, to finally answer Art's question: The faster hike will require more energy.

    (He already intuitively knew this, now he has a cerebral explanation too. *smile*)

    How much more energy? Among other things, that would depend on Art's engine (= his state of physiological fitness) and his efficiency of technique (greater efficiency = greater reduction of parasitic loss)

    Hopefully this helps.

    Need more info?
    Google: "walking gait cycle accelerations decelerations"
    if more information is needed.

    #1940096
    Jeremy and Angela
    BPL Member

    @requiem

    Locale: Northern California

    Uh, plain English?

    Hah! Now I'm worried I missed something given the numbers I got. But in simpler terms, I think the work needed to get moving is greater with increased speed, and the stopping doesn't restore any lost calories. (i.e. no Prius-style regenerative braking.)

    #1940099
    Franco Darioli
    Spectator

    @franco

    Locale: Gauche, CU.

    I just love all those formulae…
    All I did to arrive to my conclusion was to think of how I go to town and back by foot.
    It takes me 50 minutes or so to get there, then I usually walk about for a couple of hours and then I walk back.
    I do all of that without having a drink, food or rest .
    Sometime I have a coffee..
    However if I sped myself up by 1/3rd I am pretty sure that at some point I would need to have a drink and eat something so that would indicate to me that I would have burnt more calories.
    nevertheless as I stated many times my thinking is not scientific at all.

    #1940100
    a b
    Member

    @ice-axe

    There is no free lunch.

    Walking faster than the mechanics of your legs can tolerate introduces kinetic losses.

    What "faster" means is subjective.

    Jardine had a section on this in his book from 1992.
    Pretty much concluded that going faster was more efficient.
    Typical engineer thinking.
    As a plumber we are forever justifying the imaginary world of the engineer to the real world.

    I think the more individual approach Nick Gatel has voiced recently is appropriate.

    Either way, you always get less energy out of any system you put energy in to.

    What lies between ->enegy in, and energy out<-, are the losses.
    Engineer or plumber.. they are in-escapeable.

    There is no free lunch.

    By the way, this is also why solar panels and wind turbines will never amount to a mere small percentage of our energy production in this or any country.
    How do you store energy when the sun does not shine or the wind does not blow?
    How do you store energy of a hiker decending a trail?

    You can spin up a turbine or flywheel but you always lose more energy than you have input to the system.(In the long term. Solar and wind turbines take 20 years to "re-pay" their investment in energy to create them. Long before that they fall well belew their stated efficiency standards)
    So solar panels and wind turbines are actually a loss for society in general but a win for the individual user.

    We can learn something from this.
    Something that relates to the energy produced by a hiker climbing a hill.
    This is not to say it is a lost cause.
    Solar and wind and hikers are a great LOCAL source of energy.

    They are not a great GLOBAL source of energy.

    Go up that hill at a speed that suits YOU and physics be damned.

    #1940103
    Jeremy and Angela
    BPL Member

    @requiem

    Locale: Northern California

    a b: Yep, I assume that the most efficient pace for anyone is that which takes the most advantage of the natural swing of leg and arm.

    I can see solar eventually becoming more useful; particularly when paired with superconductor technology to store and distribute the power. (I think that's still quite far in the future.) There's a lovely chart of US energy flows up at https://flowcharts.llnl.gov/ that also shows how much is lost in transit.

    #1940144
    Jerry Adams
    BPL Member

    @retiredjerry

    Locale: Oregon and Washington

    "By the way, this is also why solar panels and wind turbines will never amount to a mere small percentage of our energy production in this or any country.
    How do you store energy when the sun does not shine or the wind does not blow?"

    That's the problem

    If you have more long distance power lines you can transfer power from where you have excess to where it's needed. If you double the voltage, and increase tower height by 50%, you will reduce losses for a 1000 mile transit from 50% to 10% (something like that – too lazy to find accurate numbers)

    If the power company had control of the bottom element of your water heater, they could turn it off when they don't have enough power.

    Solar power units that melt some medium like sodium, could then store it for hours until power is needed, then use it to create steam to create electricity.

    Big air conditioning users could freeze water during the night when there's excess power, and then use it to cool the building when there's a shortage of electricity.

    If we eliminated all the subsidies that fossil fuels have got, from our "best government money can buy", then alternate energy sources would be more viable.

    #1940154
    Nick Gatel
    BPL Member

    @ngatel

    Locale: Southern California

    "By the way, this is also why solar panels and wind turbines will never amount to a mere small percentage of our energy production in this or any country.
    How do you store energy when the sun does not shine or the wind does not blow?"

    That's the problem



    One other thing. When solar and wind farms are not producing energy, they need energy from the grid to stay active. They don't just shut down. Wind farms use something like 20% of their output when off line.

    You cannot take conventional power plants off line permanently because you need their capacity when wind and solar are not producing.

    The mammoth Ivanpah Solar System in California will use the sodium technology Jerry discussed. Only it covers 4,000 acres of land, which will no longer be able to absorb CO2 or support the life it once did. An ecological disaster — there is no free lunch.

    My rant can be found here: Green Greed

    #1940162
    Jerry Adams
    BPL Member

    @retiredjerry

    Locale: Oregon and Washington

    Ha ha ha – good to have Nick back : ) I've read your rant. Like the Don Quixote analogy…

    Since this thread is "question for you physics experts" and it's already degenerated to chaff even if not officially there…

    According to http://en.wikipedia.org/wiki/Cost_of_electricity_by_source

    Clear winner is – cost of natural gas is about $70 per MWh if you use a "combined cycle" – whatever that is – and I think since the cost of natural gas is dropping it's even better

    Then hydro is $90 – but we've dammed all possible rivers so we can't increase this

    Then there's a tie for about $100 between coal, geothermal, and wind turbines

    Nuclear is $110

    But then you have to factor in subsidies

    We allow coal to produce huge amount of pollution which kills people – we should tax to pay for this. And getting the coal out of the ground destroys all that land in Appalacia and other areas.

    The government did all the research to develop nuclear in the first place, and we exempt them from liability for leaks – if we ended that all nuclear plants would immediately close

    What the government should do is figure out how to better use all that natural gas that we're discovering because of fracking. We could use it to power vehicles instead of gasoline from Saudia Arabia.

    But we'll run out of this eventually, so we should develop alternatives in parallel so we will have solution to storage problem.

    Plus, as we discover how bad global warming is going to be, even the natural gas which produces less CO2 than other fossil fuels, will become unviable.

    #1940164
    Art …
    BPL Member

    @asandh

    I really enjoy learning about wind and solar energy, but since I doubt I'll be running with these technologies on my back … maybe in a different thread.

    I appreciate all the great input so far, thanks.

    #1940168
    Jerry Adams
    BPL Member

    @retiredjerry

    Locale: Oregon and Washington

    "I really enjoy learning about wind and solar energy, but since I doubt I'll be running with these technologies on my back … maybe in a different thread."

    Sorry Art – I couldn't help myself responding to someone else's misinformation : )

    To answer your question, it's intuitively obvious that it takes more energy to go the same distance at a faster rate.

    It's a physiology problem too difficult to solve using engineering/physics

    If you look at advice for loosing weight, they say you loose more weight running faster. Like, they measure the amount of CO2 exhaled while running on a treadmill. I don't have a source, I just vaguely remember from somewhere – maybe PBS.

    Amount of CO2 exhaled… that takes us back to global warming…

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