Author Topic: Power, Energy, and food  (Read 2521 times)

quixoticgeek

  • Mostly Harmless
Power, Energy, and food
« on: January 22, 2018, 10:50:07 am »

I'm trying to understand the relationship between power, and energy. I've tried playing with the maths/physics, but the numbers I'm getting aren't consistent and often seem implausible. So I'm wondering if anyone here could help me get a better understanding of how this works.

If a 100kg rider + bike, were to climb 100m of height, up a 10% incline. How much total energy would be needed?

To do the climb at 10kph, how much power would be needed?

Assuming no headwind?

Thanks

J
--
Beer, bikes, and backpacking
http://b.42q.eu/

Wowbagger

  • Dez's butler
    • Musings of a Gentleman Cyclist
Re: Power, Energy, and food
« Reply #1 on: January 22, 2018, 10:55:57 am »
1 large slice of chocolate cake, I reckon.
Oh, Bach without any doubt. Bach every time for me.

quixoticgeek

  • Mostly Harmless
Re: Power, Energy, and food
« Reply #2 on: January 22, 2018, 10:57:10 am »
1 large slice of chocolate cake, I reckon.

Metric or Imperial slice? Yesterday I realised I was cycling along at a rate of 2 mars bars per hour...

J
--
Beer, bikes, and backpacking
http://b.42q.eu/

sib

Re: Power, Energy, and food
« Reply #3 on: January 22, 2018, 11:35:12 am »
from analyticcycling.com i'd estimate 290W - assuming i got the parameters right :)

Frontal Area   0.50   m2
Coefficient Wind Drag   0.50   dimensionless
Air Density   1.226   kg/m3
Weight   100.0   kg
Coefficient of Rolling   0.004   dimensionless
Grade   0.100   decimal
Wind Resistance   1.2   kg m/s2
Rolling Resistance   3.9   kg m/s2
Slope Force   98.1   kg m/s2
Cadence   100.   rev/min
Crank Length   170.   mm
Pedal Speed   1.78   m/s
Average Pedal Force   162.3   kg m/s2
Effective Pedaling Range   70.   degree
Effective Pedal Force   417.3   kg m/s2
Speed   2.80   m/s
Power   288.9   watts

sib

Re: Power, Energy, and food
« Reply #4 on: January 22, 2018, 11:39:23 am »
and that would be c. 1044 kcal per hour - about 4 mars bars !

Samuel D

Re: Power, Energy, and food
« Reply #5 on: January 22, 2018, 11:40:16 am »
Potential energy = mass × g × height. So the rider would gain 100 × 9.81 × 100 = 98100 joules on the climb.

With a 10% slope, the 100 m tall hill would have a run of 1000 m. But the slope length would be fractionally longer according to Pythagorean’s theorem. The distance ridden would therefore be the square root of (100^2 + 1000^2) or 1005 m. Okay, not much difference.

10 km/h is 2.77 m/s, so covering the 1005 m would take 362 s.

A watt is defined as a joule per second (SI units are beautiful), so expending 98100 J in 362 s would require 271 W. That’s the power needed to drag the weight up the hill against gravity at that speed.

But rolling a bicycle on flat ground at 10 km/h takes in the region of 20 W according to Bike Calculator, so the total power for climbing the hill at 10 km/h on a still day would be very close to 290 W. That’s likely more than you can sustain for six minutes.

A food calorie is about 4.2 kJ. As calculated above, this climb would require in the region of (362 s × 290 W) = 105000 J, which is therefore about 25 calories. However, humans are only about 25% efficient at converting food energy to work done, so you’d need to eat about four times that or 100 calories to replace the energy used on the climb.

EDIT: in short, I agree with sib. Maybe my working out was useful.

sib

Re: Power, Energy, and food
« Reply #6 on: January 22, 2018, 11:41:31 am »
and explanation of power > joules > calories here..
http://mccraw.co.uk/powertap-meter-convert-watts-calories-burned/

Cudzoziemiec

  • Solar powered, tea fuelled cycle-wol
Re: Power, Energy, and food
« Reply #7 on: January 22, 2018, 11:44:10 am »
1 large slice of chocolate cake, I reckon.

Metric or Imperial slice? Yesterday I realised I was cycling along at a rate of 2 mars bars per hour...

J
A slice that your Granny would cut for you on your day off from school.
An ungovernable laughter, a joyous agitation which makes the summer stretching before you seem like an unrolling canvas on which you might draw those first rude pure strokes that are free. (Capote)

sib

Re: Power, Energy, and food
« Reply #8 on: January 22, 2018, 11:48:09 am »
EDIT: in short, I agree with sib. Maybe my working out was useful.

and much more eloquent  :thumbsup:

simonp

  • Omnomnomnipotent.
Re: Power, Energy, and food
« Reply #9 on: January 22, 2018, 12:00:09 pm »
I don't think anyone has discussed efficiency of conversion from input energy (food) to output (power).

A calorie is roughly 4.2 Joules.

If you work at 100W you are generating 100 Joules of energy every second. This means that in 1 hour you produce 360kJ of work.

However, the human body is only around 25% efficient at converting food energy to work - so you have to multiply by 4.

However since there is also roughly a 4:1 ratio between calories and Joules, it happens that it comes out as about 360 kilocalories per hour for 100W.

Note that when food energy is discussed generally people mean kilocalories when they say calories.


Zed43

  • prefers UK hills over Dutch mountains
Re: Power, Energy, and food
« Reply #10 on: January 24, 2018, 01:53:11 pm »
I don't think anyone has discussed efficiency of conversion from input energy (food) to output (power)
Do you mean the conversion from Joules (calories) in the food to energy in the body or the fact that your body burns more Joules (into heat mostly) than gets transferred onto the pedals? Or both?

Regarding the first, if you ingest a) 100 cal of gel bar, b) 100 cal of Mars bar c) 100 cal of dark rye bread, will you end up with the same amount of additional energy (leaving aside that the energy from the gel bar is probably available faster than from bread).

Interesting topic, I'm still trying to find the balance in how much food I need during a long ride. Having a full meal at each and every control at LEL was nice, but unnecessary; when I got home I noticed I had gained 2 pounds  :o

mattc

  • n.b. have grown beard since photo taken
    • Didcot Audaxes
Re: Power, Energy, and food
« Reply #11 on: January 24, 2018, 07:16:59 pm »
I think Simon was pretty clear on that point:

...
However, the human body is only around 25% efficient at converting food energy to work - so you have to multiply by 4.
...

I would add that IIRC some articles/books read yeeeeears ago; yes, we do convert energy from meat/fish/sugar at about the same efficiency. (or close enough for the sort of uses human beings have for the figures!)
Has never ridden RAAM
---------
No.11  Because of the great host of those who dislike the least appearance of "swank " when they travel the roads and lanes. - From Kuklos' 39 Articles

hellymedic

  • Just do it!
Re: Power, Energy, and food
« Reply #12 on: January 27, 2018, 10:07:16 pm »
What matters is how you can access energy stores.
We all have enough energy for any ride stored as fat.
Some folk, through training, keto-adaptation or good luck can access and and use this almost bottomless [sic] source of energy. A day of cycling would 'burn' around a pound of fat - 3500 kcal or about 15MJ.

Folk who can't mobilise their fat stores will depend on ingested food, possibly mostly carbohydrate, to keep moving.

People vary enormously in the way they handle foods. Whilst energy requirements will be broadly similar between individuals, the sorts of food they need to eat will vary.

Weight gain after LEL won't be entirely fat as water follows carbohydrate movements.

quixoticgeek

  • Mostly Harmless
Re: Power, Energy, and food
« Reply #13 on: January 30, 2018, 06:59:45 pm »
What matters is how you can access energy stores.
We all have enough energy for any ride stored as fat.
Some folk, rough training, keto-adaptation or good luck can access and and use this almost bottomless [sic] source of energy. A day of cycling would 'burn' around a pound of fat - 3500 kcal or about 15MJ.

1gm of body fat is roughly 9kcal of energy. I worked out that I could cycle about 10800km on my current fat reserves, and still end up with a healthy BMI...

If only it was that simple...

J
--
Beer, bikes, and backpacking
http://b.42q.eu/

Cudzoziemiec

  • Solar powered, tea fuelled cycle-wol
Re: Power, Energy, and food
« Reply #14 on: January 30, 2018, 07:47:55 pm »
"Not more than 30 miles a meal is sound practice," according to Cycling magazine in December 1930.
An ungovernable laughter, a joyous agitation which makes the summer stretching before you seem like an unrolling canvas on which you might draw those first rude pure strokes that are free. (Capote)

hellymedic

  • Just do it!
Re: Power, Energy, and food
« Reply #15 on: January 30, 2018, 08:47:06 pm »
What matters is how you can access energy stores.
We all have enough energy for any ride stored as fat.
Some folk, rough training, keto-adaptation or good luck can access and and use this almost bottomless [sic] source of energy. A day of cycling would 'burn' around a pound of fat - 3500 kcal or about 15MJ.

1gm of body fat is roughly 9kcal of energy. I worked out that I could cycle about 10800km on my current fat reserves, and still end up with a healthy BMI...

If only it was that simple...

J

That's why I stressed that access to this was crucial...

Caffeine is said to improve mobilisation of body fat and some say that fat burns with carb kindling.

simonp

  • Omnomnomnipotent.
Re: Power, Energy, and food
« Reply #16 on: January 30, 2018, 11:18:49 pm »
The place where I had RQ testing told me that they made a guy bonk then fed him carbs. Adding carbs at this point increased his work rate and increased fat burning.

hellymedic

  • Just do it!
Re: Power, Energy, and food
« Reply #17 on: January 31, 2018, 03:21:08 pm »
For maximum power, you need to fire on all cylinders IMO:
Ingested sugars
Stored glycogen in liver and muscles
Ingested fat
Visceral fat
Subcutaneous fat.

For low-power states, like desk-jockeying, just about anything will do and extrinsic carbs aren't needed.

Samuel D

Re: Power, Energy, and food
« Reply #18 on: January 31, 2018, 03:39:00 pm »
Can the brain run on fat? Mine can’t! Bobby Fischer used to sip orange juice while playing chess.

hellymedic

  • Just do it!
Re: Power, Energy, and food
« Reply #19 on: January 31, 2018, 05:28:35 pm »
Not really and not well.
Put it this way, an insulin overdose can cause irreversible brain damage...

A low blood sugar can mimic drunkenness and cause unconsciousness.

Most people can make or release enough glucose to keep their brain ticking over in the starved state but some they can get rather nasty if the blood glucose drops.

hellymedic

  • Just do it!
Re: Power, Energy, and food
« Reply #20 on: January 31, 2018, 10:31:46 pm »
About half the energy in a bog standard CAEK (equal weights of sugar, flour, oil and egg) will be from fat/oil and a quarter will be from sugar. The fat will delay sugar absorption and provide its own energy.

CAEK is not the same, metabolically as plain sugar.
It is still too sweet for some folk at times.

Kim

  • Timelord
Re: Power, Energy, and food
« Reply #21 on: January 31, 2018, 10:33:18 pm »
There's a lot to be said for a Mk 1 cheese sandwich...
To ride the Windcheetah, first, you must embrace the cantilever...

Samuel D

Re: Power, Energy, and food
« Reply #22 on: February 01, 2018, 01:02:25 am »
In Romania last summer I helped a man cover a grave with gravel and lots of it. It was a job with shovels and a wheelbarrow.

He started so slowly that it annoyed me, since I wanted to get the job done and be gone. An hour later I had come around to thinking his pace made sense in the heat. Two hours, and the scale of the job was dawning on me. At three hours and barely halfway, I was forced to stop and find food.

When I returned he was chugging along at the same, steady rate, regularly pausing for a drink in the shade.

After more work we were done, and I could hardly stand. He seemed not much worse for wear.

I know this man and his habits. (I rode his bicycle.) He eats two large, fatty, meat-centric meals a day, which typically include much bread. He doesn’t snack at all.

It made an impression on me anyway.

Re: Power, Energy, and food
« Reply #23 on: February 01, 2018, 09:06:41 am »
The brains preferred substrate is glucose.  However for those practising fasting and LCHF diets the brain can quite happily also utilise ketones.

We are all wedded to the idea that insulin is a sugar regulator but whilst it does that in our simple carbohydrate culture in most populations it acts as a fat storage hormone in times of plenty.

this is a counter intuitive way of looking at insulin but the most potent action of insulin is to maximise fat storage and it does this by driving "spare" FFA and glucose into the fat cells and out of the bloodstream.