A Question for You Physics Experts
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Art ...
(asandh) - F
A Question for You Physics Experts on 12/27/2012 10:06:54 MST Print View

If I hike/run up and down a mountain in 4 hours,
and then later hike/run the exact same route in 3 hours,
am I expending more, less, or the same amount of energy each time, even though I am more tired after the 3 hour effort.

I seem to recall a theory from economics class years ago ...
"Speed of Adjustment",
that stated it is always more expensive to do something more quickly.
Does this apply here ?

Edited by asandh on 12/27/2012 10:08:15 MST.

Hiking Malto
(gg-man) - F
In a perfect world on 12/27/2012 10:24:07 MST Print View

It would be the exact same energy........ However I suspect that you would increase the energy in the faster mode because the mechanics of your motion may become less efficient.

Lyan Jordan
(redmonk)

Locale: Greater Yellowstone Ecosystem
A Question for You Physics Experts on 12/27/2012 10:26:27 MST Print View

short answer : in an ideal world where nothing but the time to perform the task changes, the same energy, but more energy per unit of time. It takes more power to due the same work faster. But, the body is not an ideal world, and the energy required increases as it uses less efficient pathways to keep the muscles going at a higher intensity.

HK Newman
(hknewman) - MLife

Locale: Western US
Re: A Question for You Physics Experts on 12/27/2012 10:30:06 MST Print View

Been awhile since I taught integrated physics and chemistry but I'll take a stab. It takes the same amount of energy to move your mass but (1) there's a time component in the power equation - delta t - and (2) your muscles will burn more Calories per hour moving faster everything else staying relatively the same (last I saw this were treadmill studies from University of Texas measuring respiration, calories, etc.. written up in a professional journal iirc). Add that I need to apply any of the above after Xmas Caloric Intake.

Edited by hknewman on 12/27/2012 10:34:11 MST.

Ryan Smith
(ViolentGreen) - F

Locale: Southeast
Re: Re: A Question for You Physics Experts on 12/27/2012 11:07:55 MST Print View

The best way that it was explained to me was this - Imagine you are crushing a Pepsi can in one of those old school crushers with the long lever. You can crush it very slowly and it takes minimal effort. Crush it as fast as you can and it takes as much force as you can generate. The added speed increases the effort you have to put into it, thus increasing the energy output.

I've also seen this measured on a chassis dyno. The higher the engine RPM and wheel speed, the higher the parasitic losses on horsepower all things being equal. Takes more horsepower to turn everything at 7k rpm as opposed to 2k rpm.

Ryan

Ben Crocker
(alexdrewreed) - M

Locale: Kentucky
What Greg said on 12/27/2012 11:34:48 MST Print View

It is generally the same amount of energy being expended when you view your body as an isolated system(except you have more kinetic energy at the very tail end of your trip because you are going faster, presumably). I suspect your body is less efficient at the higher speeds too, which involves more energy burned by your body but being converted to heat instead of energy needed to move your body.

If you look at it from a more pure standpoint, there has been no net change in energy states if you go up and come back down to the same place and stop. Really, all the energy your body has burned has been blown on friction and heat and you energy state is the same since you are back in the same place.

Greg Mihalik
(greg23) - M

Locale: Colorado
Re: A Question for You Physics Experts on 12/27/2012 11:45:05 MST Print View

"... am I expending more, less, or the same amount of energy each time ..."

First of all, reading a little more into your question -

If you are asking about Work-

it's Mass over Distance - and Work stays the same. Same mass, same distance.

If you really are asking about Energy it gets a lot more complex. Your physiology comes into play, and the answer is "More".

People aren't efficient and straightforward about converting energy into work. A large amount of calories expended go into heat (just like an incandescent light bulb). As effort goes up utilization, and efficiency, may go down.

Depending on output level, our energy source may change from one metabolic source to another, which may affect your overall efficiency. Some people (elite athletes) maintain their efficiency during very high levels of output. Others, not so much.

And as Greg G. mentions, we may become less efficient mechanically just due to the difference in the mechanics of our stride, arm swing, posture, etc., which means more energy is needed.

YMMV

p.s. Not an expert in Any field.

Edited by greg23 on 12/27/2012 11:52:36 MST.

Ben Crocker
(alexdrewreed) - M

Locale: Kentucky
Work on 12/27/2012 12:08:32 MST Print View

Work is force x distance. Mass may have a role in that.

Art ...
(asandh) - F
Re: Work on 12/27/2012 12:18:07 MST Print View

so I'm not really sure now if I'm asking about work or energy.
are these 2 the same in a perfect world with no inefficiencies ?

thus with no inefficiencies time is not relevant to my question because I am simply moving the same mass from A to B each time ?

and it is only the fact that inefficincies develop as I speed up that causes the shorter time hike to use more energy ?

Greg Mihalik
(greg23) - M

Locale: Colorado
Re: Re: Work on 12/27/2012 12:34:27 MST Print View

"are these 2 the same in a perfect world with no inefficiencies ?"

yep.



"and it is only the fact that inefficincies develop as I speed up that causes the shorter time hike to use more energy ?"

Yep, for you, in this scenario.

Ben Crocker
(alexdrewreed) - M

Locale: Kentucky
Work and energy on 12/27/2012 12:39:20 MST Print View

It does take the same amount of work to go from point A to point B assuming no lost energy to heat,friction, etc. Its just being done in a shorter amount of time.

Truly, though, if you wind up in the same spot and there is nothing lost to friction or heat, it would take no energy. Walking is virtually all friction/heat/ lost energy unless you are going up hill. At least if you consider the body as a whole system. If we were truly efficient, we would glide like an air hockey puck (even a little better)and it wouldn't take any energy to travel, except when going uphill. When we come back downhill, we would gain that work back.

Greg Mihalik
(greg23) - M

Locale: Colorado
Re: Work and energy on 12/27/2012 12:42:14 MST Print View

"Truly, though, if you wind up in the same spot and there is nothing lost to friction or heat, it would take no energy."

If only this Were true...

I think you meant Work. But that is only for the specialized cases involving conservative force fields. Otherwise it is the "force along the path", and is certainly not zero.

Edited by greg23 on 12/27/2012 13:34:37 MST.

Jeremy B.
(requiem) - F - M

Locale: Northern California
Re: Work and energy on 12/27/2012 14:15:37 MST Print View

If we were truly efficient, we would glide like an air hockey puck (even a little better)and it wouldn't take any energy to travel, except when going uphill.

Here's where I'll toss in Newtons 1st and 2nd laws. Assuming you are starting the hike from a standstill, and want to stop at the destination (rather than glide right past), you'll need to exert a force to both start and stop. (F=ma) Note that in this case, since acceleration has a time component, more force will be needed to achieve faster transit times.

Ben Crocker
(alexdrewreed) - M

Locale: Kentucky
Work and energy on 12/27/2012 14:32:38 MST Print View

Good point Jeremy, except for one thing. In deceleration, the force and distance are in opposite directions. This means that you actually could capture energy by stopping a moving object(the hiker in this case) in the exact same amount that was spent accelerating the object(the hiker)and the work involved is still the same(under the big assumptions of no energy lost to friction, heat, drag, etc.). Its zero.

You can actually have a braking system that captures energy.

Todd Taylor
(texasbb) - F

Locale: Pacific Northwest
My contribution on 12/27/2012 16:54:05 MST Print View

This has gone off in ways that are useless and interesting. :) Here's my contribution to both:

It takes the same amount of work to haul your carcass up and down the mountain no matter how fast you move.

The energy you burn to do that work differs because of differences in efficiency between the slow and fast trips:

* Friction between your feet and the ground is different. I'd guess running results in more slippage, hence lower efficiency.

* Your body's mechanical efficiency is probably lower when walking. We're smoother runners than walkers, at least when we're working hard at both.

* Your body's metabolic (right word?) efficiency is probably lower when running. I'm guessing at this one too, but the necessity of sweating and such probably lowers efficiency.

Jeffs Eleven
(WoodenWizard) - F

Locale: Greater Mt Tabor
Re: My contribution on 12/27/2012 19:23:05 MST Print View

This came up at the Olympics. They were saying Usain Bolt was doing less work because in the 50 he takes like 43 steps while others avg 49 steps. (numbers are estimates, but they are close IIRC)

Greg Mihalik
(greg23) - M

Locale: Colorado
Re: Work versus Energy on 12/27/2012 20:38:44 MST Print View

"They were saying Usain Bolt was doing less work because in the 50 he takes like 43 steps while others avg 49 steps."


41 strides may require less Energy than 45 strides. (He is more efficient.)

But if both athletes weighed the same the Work would be the same.

Stuart R
(Scunnered) - F

Locale: Scotland
Energy: rule of thumb on 12/28/2012 02:22:02 MST Print View

There is a rule of thumb (ie an approximation) for energy expenditure that some runners may be familiar with:

Energy expended (kCal) in running from A to B = body mass (kg) x Distance (km)

To a first approximation, there is no dependency on speed.
The implication is that the maximum speed that you can run any particular distance (ignoring sprint distances) will depend on your maximum oxygen consumption (VO2max) (a measure of sustained power capability), weight and running efficiency.

I suspect the equation will hold true for walking on level ground too, but obviously not for hiking/running up and down, where potential energy would have to be taken into account.

Jennifer Mitol
(Jenmitol) - M

Locale: In my dreams....
The human machine... on 12/28/2012 11:20:29 MST Print View

If we were speaking of a mechanical machine, you could perform all these fabulous calculations and whatnot to arrive to a conclusion. The human body, however, has many, many more variables than mass, energy, work, etc.

There are so many other systems going at the same time in your body and they will all have an effect...under some conditions the 4 hour ascent will take more energy, under others the faster ascent will. What is your immune system doing? How efficient was your digestion before you started your ascent? Did you expend energy yesterday and have a depleted supply? This would mean drawing on more reserves, which consumes more energy to perform the same task (think the cumulative challenge of the Tour de France)
Anyway, as much as I truly love a good physics discussion, there is a great deal more to your question....

Franco Darioli
(Franco) - M

Locale: Melbourne
A Question for You Physics Experts on 12/28/2012 15:50:23 MST Print View

In my very unscientific/untrained mind what comes up is that it would depend a lot on how close the shorter time is to your best possible time.
To explain if, say ,I take 20 sec to run 100m (at my very best) I am pretty sure that taking 26 sec (about 30% longer) I would spend less energy as well as feeling less tired .
Now if I walked the 100m in 60 second or 90 it would hardly matter.
( no I have not tested that, it"s just the way I think )

Stuart R
(Scunnered) - F

Locale: Scotland
Re: A Question for You Physics Experts on 12/29/2012 04:10:55 MST Print View

Franco - in terms of energy used, it does not make a lot of difference whether you run or walk 100m. How tired you feel afterwards is a totally different question. That will depend on how much of the energy source is used (ATP, glucose, glycogen, or fatty acids), how well trained you are at a particular speed and a host of other factors.

Jennifer Mitol
(Jenmitol) - M

Locale: In my dreams....
Running efficiency on 12/29/2012 07:28:45 MST Print View

There is an actual calculation that we can do in the lab that calculates something called running efficiency. Much of it is genetic, but it can be improved with training. Certain gait styles, shoes, ambient temps, etc can all alter running efficiency and this may be more to the point of your original question....

Chris W
(simplespirit) - MLife

Locale: .
Re: Running vs walking on 12/29/2012 07:32:52 MST Print View

Short story - it's roughly the same if the only variable is speed. You can get as technical as you want, but the outcome won't change.

Diplomatic Mike
(MikefaeDundee)

Locale: Under a bush in Scotland
Fuel used? on 12/29/2012 08:21:16 MST Print View

You use more fuel if you drive at 100mph than if you drive at 60mph. :-)

Chris W
(simplespirit) - MLife

Locale: .
Re: Fuel used? on 12/29/2012 11:38:53 MST Print View

Depends on what the vehicle is geared for. You can gear for max efficiency at 60 or 100.

Diplomatic Mike
(MikefaeDundee)

Locale: Under a bush in Scotland
RE Fuel used on 12/29/2012 12:12:18 MST Print View

Same vehicle. :-)

Nelson Sherry
(nsherry61)

Locale: Mid-Willamette Valley
Shifting gate and metabolism effecting energy use on 12/29/2012 13:43:40 MST Print View

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. :)

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: Shifting gate and metabolism effecting energy use on 12/29/2012 13:50:00 MST Print View

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

No. How about gait?

--B.G.--

Jennifer Mitol
(Jenmitol) - M

Locale: In my dreams....
"Gate" mechanics... on 12/29/2012 20:23:32 MST Print View

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.

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Shifting gate and metabolism effecting energy use on 12/29/2012 21:14:06 MST Print View

"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

Greg Mihalik
(greg23) - M

Locale: Colorado
Re: Wettness as a function of speed on 12/29/2012 21:36:41 MST Print View

"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.

Edited by greg23 on 12/29/2012 21:39:05 MST.

Franco Darioli
(Franco) - M

Locale: Melbourne
A Question for You Physics Experts on 12/29/2012 22:36:29 MST Print View

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.

Edited by Franco on 12/29/2012 23:12:43 MST.

Art ...
(asandh) - F
Re: A Question for You Physics Experts on 12/30/2012 00:29:42 MST Print View

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 ...

Edited by asandh on 12/30/2012 08:46:58 MST.

William Segraves
(sbill9000) - F - M
Efficiency on 12/30/2012 05:53:29 MST Print View

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

Stuart R
(Scunnered) - F

Locale: Scotland
Re: A Question for You Physics Experts on 12/30/2012 09:43:46 MST Print View

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

Ben Crocker
(alexdrewreed) - M

Locale: Kentucky
run or walk on 12/30/2012 10:22:20 MST Print View

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.

Greg Mihalik
(greg23) - M

Locale: Colorado
Re: run or walk on 12/30/2012 11:02:55 MST Print View

"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.

Edited by greg23 on 12/30/2012 11:04:22 MST.

Nick Gatel
(ngatel) - MLife

Locale: Southern California
Re: A Question for You Physics Experts on 01/01/2013 21:04:50 MST Print View

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.

Jeremy B.
(requiem) - F - M

Locale: Northern California
Re: run or walk on 01/01/2013 23:34:48 MST Print View

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".

Edited by requiem on 01/01/2013 23:51:44 MST.

Nick Gatel
(ngatel) - MLife

Locale: Southern California
Re: Re: run or walk on 01/01/2013 23:48:41 MST Print View

"So, what am I missing?"


Uh, plain English?

;)

Tony Ronco
(tr-browsing) - MLife
Re: A Question for You Physics Experts on 01/02/2013 00:03:47 MST Print View

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.

Edited by tr-browsing on 01/02/2013 00:30:15 MST.

Jeremy B.
(requiem) - F - M

Locale: Northern California
Re: Re: Re: run or walk on 01/02/2013 00:30:54 MST Print View

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.)

Franco Darioli
(Franco) - M

Locale: Melbourne
A Question for You Physics Experts on 01/02/2013 00:55:59 MST Print View

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.

a b
(Ice-axe)
Re: Re: Re: Re: run or walk on 01/02/2013 00:56:34 MST Print View

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.

Edited by Ice-axe on 01/02/2013 01:03:16 MST.

Jeremy B.
(requiem) - F - M

Locale: Northern California
Re: Re: Re: Re: Re: run or walk on 01/02/2013 01:26:25 MST Print View

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.

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Re: Re: Re: Re: run or walk on 01/02/2013 08:39:43 MST Print View

"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.

Nick Gatel
(ngatel) - MLife

Locale: Southern California
Re: Re: Re: Re: Re: Re: run or walk on 01/02/2013 09:01:48 MST Print View

"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

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Re: Re: Re: Re: Re: Re: run or walk on 01/02/2013 09:26:38 MST Print View

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.

Art ...
(asandh) - F
Re: Re: Re: Re: Re: Re: Re: Re: run or walk on 01/02/2013 09:30:21 MST Print View

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.

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Re: Re: Re: Re: Re: Re: Re: Re: run or walk on 01/02/2013 09:40:32 MST Print View

"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...

Tony Ronco
(tr-browsing) - MLife
Re: Re: Re: Re: Re: Re: Re: Re: Re: run or walk on 01/02/2013 10:10:29 MST Print View

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

Engineering and Physics are two foundational cornerstones for physiology. The physical science of physiology can not exist independently without them.

-------------------

As previously stated: there is no free lunch.
The faster you go, the more prominent the parasitic loss of energy.

Edited by tr-browsing on 01/05/2013 06:24:36 MST.

M B
(livingontheroad) - M
power on 01/08/2013 15:53:34 MST Print View

Work=Force*Distance
Power = work/time
Force=mass*acceleration (= weight here basically)

One of the most difficult things to accept for some people , is that with no D, there is no work. I.e. you can struggle and sweat to hold up 100 lbs until you collapse. But if you dont move it, you havent done any work on it. The faster you move it thru the same distance, the higher you power output is, but the same work is being done.

If you have 100 lb wt on wheels, and roll it horizontally, did you do any work?
In a frictionless world, Nope. Even though you might move it, the only resistance to it moving horizontal is friction. In reality you know that may be a substantial amount depending on how good the wheels are and what the surface is. Same applies when your body is the "wheels", its called innefficiency.

The real base work you do would be lifting your body weight , the vertical distance you lifted it by going uphill. That doesnt change, its the same in both cases.

You need a higher power output to do so in a shorter time, thats why you are more tired.