Re: Work and energy

Peter <Poakfield@xxxxxxx> wrote in message
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On Aug 22, 1:14 pm, "Androcles" <Headmas...@xxxxxxxxxxxxxxxxxx> wrote:
"Peter" <Poakfi...@xxxxxxx> wrote in message

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On Aug 21, 12:33 pm, "Androcles" <Headmas...@xxxxxxxxxxxxxxxxxx>
wrote:





"Peter" <Poakfi...@xxxxxxx> wrote in message

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On Aug 21, 8:29 am, "Androcles" <Headmas...@xxxxxxxxxxxxxxxxxx> wrote:

"Peter" <Poakfi...@xxxxxxx> wrote in message

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Hi! Exerting a constant force, we can lift a barbell to a certain
height in either two seconds or two minutes. The final potential
energyof the barbell is the same in both cases, but theenergywe
spend in the second case is much greater than in the first.Why?

Lo!
No it isn't, Poakfield. Why?
Hint: Learn about power.

If you do not believe me, try lifting the barbell, first taking two
seconds, and then two minutes. You will feel the difference.
==========================================
I do not believe you and I'm not that stupid, Poakfield.
Anyway, I can't, lifting weights would raise my blood pressure,
burst my aorta and kill me.
http://www.web-books.com/eLibrary/Medicine/Cardiovascular/Images/aneu...

Energyisn't power. Why won't you learn about power, Poakfield?

I don't want you to die. Please then answer this question: Suppose a
car accelerates from zero to 60 mph in one minute. If the same car is
made to accelerate from zero to 60 mph in ten minutes, is the amount
of gasoline the car consumes exactly the same in both cases? Forget
about air drag.
================================================
Car engines are notoriously inefficient, 82% of theenergyis wasted as
heat (which radiates and is lost), never converted to usefulwork.
The exhaust is hot, the engine is water cooled.
In the situation you describe the engine would run for 9 minutes
longer than necessary to accomplish the desiredwork, all the wastedenergybeing converted to heat.
So no, the amount of gasoline used will not be the same.
Efficiency iswork/energy* 100%, but "work" means usefulwork.
If the heat is used to warm the car's interior on a cold day then
the efficiency is raised, we've redefined "work".
If used for air-conditioning on a hot day, you'll burn more fuel
driving the AC compressor and it will not do anything for the mechanical
efficiency.

For your further edification, a fossil-fuelled power station of 1980s
vintage on the north bank of the Medway that I know of is 34% efficient.
I can assure you that if the engineers could improve on that by just 1%
they would do so.
High pressure steam enters a turbine which turns the blades... great,
but the steam still has a medium pressure after that. So it is then used
to turn bigger blades, and it still has a lower pressure after that...
so it is used to turn even bigger blades.
http://tinyurl.com/mogv2k

So while a transformer might be 99% efficient, losing 1% to heat,
a turbine generator from fossil fuel (or nuclear fuel) is much worse.

Years ago I was working on an autothrottle for Boeing's 747 jumbo.
The computer achieved a saving over human direct control of the
engines of just 0.5%, which doesn't sound like much but the plane
burns two tons of fuel on take-off alone. Saving fuel saves money,
so the airlines and Boeing wanted the box of electronics and I got
paid.

So if you know what power is, now you know what efficiency is,
and all you need do isworkout how many bowls of wheaties it
takes to lift your shopping bag up to the penthouse, how much
you'll sweat doing it as you radiate heat and find out how efficient
your muscles are. Then you'll know why it takes more effort if you
walk up instead of running up, while I take the elevator and stay
cool. We've done the samework, my shopping bag weighs
the same as your shopping bag and has been raised to the same
height. But I used electricalenergywhich came from a steam turbine
which was heated by burning fossil wheaties which grew in sunlight
and is much more inefficient than you carrying the bags, I used
far more wheaties that you.- Hide quoted text -

- Show quoted text -

Interesting. Thank you. It is clear to me that, regardless if it is a
person or a machine that lifts an object, it takes less energy if the
lifting is done in a shorter time, because both people and machines
emit heat while doing the lifting; thus, the longer it takes to lift
the object, the more energy is expended.
Peter

Not necessarily.
Yes, accelerating a car from 0 to 100 Km/h in 10 minutes takes
more gasoline than doing it in 1 minute because you are heating
up the car more.
But now ask yourself, will doing it in 1 second take less than
doing it in 1 minute?

Dirk Vdm
.

Relevant Pages

  • Re: Work and energy
    ... Car engines are notoriously inefficient, ... longer than necessary to accomplish the desiredwork, all the wastedenergybeing converted to heat. ... the efficiency is raised, we've redefined "work". ... a turbine generator from fossil fuel is much worse. ...
    (sci.physics)
  • Re: Work and energy
    ... car accelerates from zero to 60 mph in one minute. ... Car engines are notoriously inefficient, ... heat, ... a turbine generator from fossil fuel is much worse. ...
    (sci.physics)
  • Re: Work and energy
    ... car accelerates from zero to 60 mph in one minute. ... Car engines are notoriously inefficient, ... longer than necessary to accomplish the desiredwork, all the wastedenergybeing converted to heat. ... a turbine generator from fossil fuel is much worse. ...
    (sci.physics)
  • Re: Work and energy
    ... 82% of the energy is wasted as ... heat, ... the efficiency is raised, we've redefined "work". ... a turbine generator from fossil fuel is much worse. ...
    (sci.physics)
  • Re: Why use fuelcells(hydrogen) to drive cars?
    ... you can make a 500 lbs car instead of a 5,000 lbs car, the needed power ... The car structure and power system can add efficiency with proper ... "waste" heat may not be enough to heat the car. ... and that car can store electricity in a small battary on which the car ...
    (sci.energy.hydrogen)
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