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51 comments on Physics in the Economy I: Physical Work

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step back on September 12, 2005 - 8:47am Permalink | Subthread | Comments top
So if we had a measure of our national, Gross Domestic Transport Load in terms of mass times our average national acceleration needs, times our average acceleration time, we would have a crude estimate of our gross domestic work need for transport purposes? (W=m*a*t)
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JD on September 12, 2005 - 9:08am Permalink | Subthread | Parent | Comments top
That wouldn't be a physical value per se, because the work need is determined by the layout of cities. The worst case would be terminal sprawl, where you have to drive from your refrigerator to your kitchen. A city like that would have an incredible transport work need, but it wouldn't really be a need.
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Stuart Staniford on September 12, 2005 - 9:50am Permalink | Subthread | Parent | Comments top
Work is force x distance, so the layout of cities would be crucial as noted by JD - it sets the amount of distance that needs to be covered. However, on the force side, most work done in moving a vehicle is actually to overcome friction - both rolling resistance within the vehicle itself and between the vehicle and the road, and air resistance. Air resistance in particular is interesting because the force rises as the square of the vehicle speed (which is why gas mileage gets so much poorer at high speeds). The force due to rolling resistance is approximately independent of speed.
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NC on September 12, 2005 - 12:09pm Permalink | Subthread | Parent | Comments top
Okay guys,

One more aspect to think of in the real world for you to calculate.  Rate of acceleration and deceleration has a huge effect on energy to do work in transportation.  It is not just moving something at 60 mph that takes energy. It is how fast one tries to get from 0 to 60 that is the biggest consumer of fuel today.

Vehicle drive trains waste energy as they wind up to bring in the torque for acceleration.  This is not overcoming friction, it is just a wasteful design that is pretty robust.

I own a Toyota Prius that tries to dampen this effect by using the electric motor for high torque, low rpm applications.  Most of the efficiency is gained this way.  But you can still get really poor fuel economy by exceeding the current flow from the battery to electric motor and inherent motor torque to the transmission limits.  You can rev. the heck out of the gas engine and not do anymore work than accelerating somewhat slower.

Conventional cars are pretty good at constant speed but efficiency drops rapidly if speed is varied.  So how does one calculate efficiency to move mass when the efficiency is highly modified by time allowed to move the mass?  And to make it more complicated you can go the same distance (say 20 miles) faster, using less energy, just by starting and stoping at slower rate but maintaining a slightly higher cruise speed.  Same horsepower but different energy used to move the same mass over the same distance at the same speed.  They now have a name for this: Eco Driving.

What does all this mean for our assumptions of how much energy it takes to do a given task?

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isaiah on September 13, 2005 - 1:15am Permalink | Subthread | Parent | Comments top
you are right to mention the acceleration, NC (i think you mean the value or quantity of acceleration, which is equivalent to the rate of change of velocity).  as you know, Work = m*a*distance, so the higher the acceleration required, the larger the work and thus the energy necessary to perform the action. minimizing acceleration (using cruise control, speeding up/down slowly, etc.) is one good way to reduce energy use.  you are always fighting the force of friction when driving, both from the air and the ground, but by adding the force needed to produce a desired acceleration, you add to your energy consumption.  and of course the rate at which you change velocity (which is acceleration) is the precise quantity in the equation.  

so the lowest possible, reasonable acceleration should be used to minimize our work budget.  this goes into the second law, but entropy is the way we define the best possible method to perform a change in state while not "wasting" any energy.   any reversible transformation is a change of state that keeps the the net (system plus surroundings) entropy change = zero.  that is, we are not adding any entropy (or waste energy) to the universe by a reversible process.  the more entropy we create, the more energy we waste (e.g. large accelerations, circuitous paths or other losses).  ideally, we would change our velocity infinitesimally slowly in order to have the lowest possible energy expenditure, and the lowest entropy change.  of course this isn't practical, but gradual changes in speed are preferred (low acceleration and deceleration) over sudden changes (high accel and decel).  regenerative braking is one way to capture much of the waste energy used in deceleration, but braking slowly helps even more, because you waste less energy to begin with.  

practically, this means you should slowly accelerate to your ideal velocity, 40-60mph, the low end for large SUV's and the high end for light, low drag cars (calculated using the lowest drag/frictional losses per distance traveled - thus gets you where you want for the least amount of fuel) and stay there until you slowly decelerate upon your destination.  

http://auto.howstuffworks.com/question477.htm

http://www.fueleconomy.gov/feg/driveHabits.shtml

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51 comments on Physics in the Economy I: Physical Work

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