Racer/Trundler Energy

DDEckerslyke said:

Hi All

Twenty or more years ago my physics teacher said something like: 'To lift
this book this far I need this much energy.' I don't recall the exact
formula but it was pretty simple (Energy = Force x Distance maybe).

You've recalled it perfectly.

Quoted message said:

At this point I thought: 'So does it take the same amount of energy to
walk upstairs as it does to run up?'

Quoted message said:

Well does it?

Pretty much I should think, probably a bit more to run up.

Quoted message said:

The point is this: if I trundle home or I race home (or if I run a marathon
or walk a marathon) am I using the same amount of energy (because it sure
doesn't feel like it) and if I'm not using the same amount of energy then
why not?

Wind resistance gets much worse the faster you go, which is why it takes
more energy to cycle faster over the same distance.

In fact, without wind (or other kinds of) resistance, it wouldn't
require any energy at all to cruise along at a constant speed on the
flat on a bike.

But if you are pushing against resistance with a force F, the energy
required to keep going is F x d. So the formula is the same. But F is
bigger the faster you go (wind just works like that for some reason).

Try also http://www.kreuzotter.de/english/espeed.htm.

Quoted message said:

We did Power = Energy/Time the next lesson and I thought that answered
my question but it doesn't because according to the E=FD formula the
Energy is the same no matter what the speed.

Yes, exactly, power has nothing to do with it.

Except it may be that the human body is less efficient at higher power
outputs. In order to provide the energy we can predict is required to
lift up a book or walk up the stairs, your gas-guzzling body actually
uses about four times as much, i.e. it is about 25% efficient. It may be
less efficient at higher power levels, or when running rather than
walking, for some physiological reaons, I don't know.

DDEckerslyke said:

Hi All

Quoted message said:

The point is this: if I trundle home or I race home (or if I run a marathon
or walk a marathon) am I using the same amount of energy (because it sure
doesn't feel like it) and if I'm not using the same amount of energy then
why not? We did Power = Energy/Time the next lesson and I thought that
answered my question but it doesn't because according to the E=FD formula
the Energy is the same no matter what the speed. Can anyone here explain

Yes. Wind resistance is the major difference between slow and fast
cycling;

The force at a different velocity differs, as wind resistance is equal
to the square of velocity (simplification: see
http://en.wikipedia.org/wiki/Wind_resistance#Drag_at_large_velocity).
Thus, going at a greater speed increases the force, but distance is
constant, so you use more energy to travel the same distance.

There are other factors other than the energy required to overcome the
higher wind resistance. When you work harder, your body works
differently, and in particular, you are burning more high quality
energy sources (glycogen) which the body has to replace at a later
time. At lower exercise levels, more fat is burned, which doesn't
require replenishing in the same way before you can exercise again.
AIUI, there are levels below which people can pretty much exercise
indefinately (within reason) but above a certain level, you will tire
relatively quickly.

Simon

Ben C said:

DDEckerslyke said:

Hi All

Twenty or more years ago my physics teacher said something like: 'To lift
this book this far I need this much energy.' I don't recall the exact
formula but it was pretty simple (Energy = Force x Distance maybe).

Quoted message said:

You've recalled it perfectly.

Quoted message said:

Quoted message said:

At this point I thought: 'So does it take the same amount of energy to
walk upstairs as it does to run up?'

Quoted message said:

Quoted message said:

Well does it?

Quoted message said:

Pretty much I should think, probably a bit more to run up.

Quoted message said:

Quoted message said:

The point is this: if I trundle home or I race home (or if I run a marathon
or walk a marathon) am I using the same amount of energy (because it sure
doesn't feel like it) and if I'm not using the same amount of energy then
why not?

Quoted message said:

Wind resistance gets much worse the faster you go, which is why it takes
more energy to cycle faster over the same distance.

Quoted message said:

In fact, without wind (or other kinds of) resistance, it wouldn't
require any energy at all to cruise along at a constant speed on the
flat on a bike.

Quoted message said:

But if you are pushing against resistance with a force F, the energy
required to keep going is F x d. So the formula is the same. But F is
bigger the faster you go (wind just works like that for some reason).

Quoted message said:

Try also http://www.kreuzotter.de/english/espeed.htm.

Quoted message said:

Quoted message said:

We did Power = Energy/Time the next lesson and I thought that answered
my question but it doesn't because according to the E=FD formula the
Energy is the same no matter what the speed.

Quoted message said:

Yes, exactly, power has nothing to do with it.

Quoted message said:

Except it may be that the human body is less efficient at higher power
outputs. In order to provide the energy we can predict is required to
lift up a book or walk up the stairs, your gas-guzzling body actually
uses about four times as much, i.e. it is about 25% efficient. It may be
less efficient at higher power levels, or when running rather than
walking, for some physiological reaons, I don't know.

The physiological reason is legs. With every step you have to
accelerate a leg forward. You need more force to accelerate it faster,
and the faster you walk or run the faster you have to throw your legs
forward.

Physicists forget this because they've been trained to simplify
things. "Let's assume the runner is a sphere" and so on.

--
Chris Malcolm [email hidden] DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/]

in message <[email hidden]>, DDEckerslyke

(') said:

The point is this: if I trundle home or I race home (or if I run a
marathon or walk a marathon) am I using the same amount of energy
(because it sure doesn't feel like it) and if I'm not using the same
amount of energy then why not?

Air drag. Aerodynamic drag increases with the square of the speed, and is a
very significant part of the energy cost of cycling. So if you want to
cycle a given distance at least energy cost, ride slowly, or use a fully
faired recumbent.

--
[email hidden] (Simon Brooke) http://www.jasmine.org.uk/~simon/
[ Disclaimer:  This is a work displacement exercise.  Please feel free ]
[ to reply either on or off group.  Expect lengthy replies until the ]
[ deadline has passed.  Thank-you for your cooperation. ]

Quoted message said:

Physicists forget this because they've been trained to simplify things.
"Let's assume the runner is a sphere" and so on.

Yeah, I know. "Take one spherical cow".

The answer to the walking/running up stairs example is that
at arrival at the top of the stairs, your body has gained exactly the same
potential energy.
It must be so - else you could construct a perpetual motion machine.
Just dive off the top step with a line attached to a pulley and generator.
Next time up the stairs walk, and do similar with the generator.
If there is a difference in the energy extracted from the system (ie. the
electricity output) you'll have a net gain of energy every two trips. (*)

Now, that doesn't mean that you expend the same effort or work going up
the stairs. Walking, you are raising your body up and gaining potential
energy. In running, you have to overcome more air resistance, as people
have said swing those limbs faster and expend energy in noise on the
stairs, plus probably heat losses in the rebound of your shoe sole
material.

(*) My reasoning is flawed here - you're burning chemical energy as an
input to the system which differs on the walking/running cycles.
But the principle holds with respect to potential energy - two bodies
raised over the same height must gain the same potential energy, else we
would have a free way of extracting energy from gravitational fields.

Simon Brooke said:

Air drag. Aerodynamic drag increases with the square of the speed, and is a
very significant part of the energy cost of cycling. So if you want to
cycle a given distance at least energy cost, ride slowly, or use a fully
faired recumbent.

Or follow the prevailing winds.

If a route is an out and back with the far end at the top of a hill is
it better for the wind to be behind you on the uphill or the downhill?

POHB said:

If a route is an out and back with the far end at the top of a hill is
it better for the wind to be behind you on the uphill or the downhill?

Downhill. You won't notice a headwind on the uphill because you're
going slowly anyway.

(I haven't thought hard about the science, but don't neglect the psychology)

-dan

Daniel Barlow said:

POHB said:

If a route is an out and back with the far end at the top of a hill is
it better for the wind to be behind you on the uphill or the downhill?

Quoted message said:

Downhill. You won't notice a headwind on the uphill because you're
going slowly anyway.

Quoted message said:

(I haven't thought hard about the science, but don't neglect the psychology)

Or the way one of our local loops work: take the hilly route out into the
Westerly, and come back along the flat with the wind in the back.

Roos

Roos Eisma said:

Daniel Barlow said:

POHB said:

If a route is an out and back with the far end at the top of a hill is
it better for the wind to be behind you on the uphill or the downhill?

Quoted message said:

Downhill. You won't notice a headwind on the uphill because you're
going slowly anyway.

Quoted message said:

(I haven't thought hard about the science, but don't neglect the psychology)

Or the way one of our local loops work: take the hilly route out into the
Westerly, and come back along the flat with the wind in the back.

Ha! I wish...
The cyclists rule applies. Whichever way you go, it's always a
headwind.

TL

In article <[email hidden]>

Daniel Barlow said:

POHB said:

If a route is an out and back with the far end at the top of a hill is
it better for the wind to be behind you on the uphill or the downhill?

Downhill. You won't notice a headwind on the uphill because you're
going slowly anyway.

(I haven't thought hard about the science, but don't neglect the psychology)

I have fond memories of a ride that involved a long and fairly hard
climb into a headwind where I found myself comfortably leading the pack
- being fairly large I was more steady than fast uphill, but the
lightweight climbers were really suffering and I was just trundling
along as usual. So I think the answer is that it depends on what sort
of rider you are.

DDEckerslyke said:

Twenty or more years ago my physics teacher said something like: 'To
lift this book this far I need this much energy.' I don't recall the
exact formula but it was pretty simple (Energy = Force x Distance
maybe). At this point I thought: 'So does it take the same amount of
energy to walk upstairs as it does to run up?'

I think there are a number of factors at work.

When you are lifting a book, there is virtually no resistance to
movement, other than the weight of the book. If you are walking or
cycling, there is resistance that slows you down regardless of whether
you're going uphill or not. That resistance increases
disproportionately with speed, so to achieve a higher speed for a
shorter time _does_ need more energy.

It may be that walking or running up a hill does only use the same
amount of energy in total, but it is clearly using it at a different
rate. Your body can process energy much more easily at a slower rate,
which is why you are less likely to feel knackered from walking slowly
up a hill compared with running up it.

--
Stevie D
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\\\\\\\__X__/////// common hedgehog since 2001 - "HedgeHugs"
___\\\\\\\'/ \'///////_____________________________________________

Stevie D said:

DDEckerslyke said:

Twenty or more years ago my physics teacher said something like: 'To
lift this book this far I need this much energy.' I don't recall the
exact formula but it was pretty simple (Energy = Force x Distance
maybe). At this point I thought: 'So does it take the same amount of
energy to walk upstairs as it does to run up?'

I think there are a number of factors at work.

When you are lifting a book, there is virtually no resistance to
movement, other than the weight of the book. If you are walking or
cycling, there is resistance that slows you down regardless of whether
you're going uphill or not. That resistance increases
disproportionately with speed, so to achieve a higher speed for a
shorter time _does_ need more energy.

It may be that walking or running up a hill does only use the same
amount of energy in total, but it is clearly using it at a different
rate. Your body can process energy much more easily at a slower rate,
which is why you are less likely to feel knackered from walking slowly
up a hill compared with running up it.

There is a physiological factor involved as well in that as you go
faster the body will start to produce adrenalin which will ultimately
have an effect of the transfer of oxygen to the blood in an attempt to
reduce your energy expenditure to a more normal level and protect the body.

There are two common triggers for adrenalin

1) Fear A short burst of energy to escape a threat
2) Thought Positive thought to increase energy output to achieve a
Specific goal.

Neither can be maintained for long periods; although the body can be
progressively trained to ignore some of the symptoms of adrenalin
production.

Sniper8052