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The Cost Of Car
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grufty jim
grufty jim
1978 posts

Re: erm....
Feb 27, 2002, 04:11
My point is that while the engine does indeed carry you from A to B; it also needs to generate the energy to carry itself (ever tried to lift a car engine? they're bloody heavy - and it doesn't stay at home when you nip down Tescos in the car) plus the massive metal - glass - plastic - etc. frame that makes the rest of the car. The total energy you expend, therefore includes the energy to move that lot - which is many times your own weight. And whilst *you* may need to be at Point B; is there any pressing need to have that ton of metal there?

The amount of acceleration needed to move any mass from rest, is expressed by Newton's second law, which can be written:

a = F/m
(where a = acceleration, F=force and m=mass)

So consequently, the higher the mass, the higher the Force required to achieve equivalent acceleration. Get into a car and you, what? octuple your mass? At least! Probably more than that. That means you'd need 8 times the force (and a consequent increase in the energy required to generate that force) to achieve the equivalent acceleration (which creates your velocity).

On the other hand, if you walk, you only need to expend the energy to carry yourself and the clothes you wear. And you admittedly achieve a far lower acceleration. All in all, you probably expend a couple of hundred times more energy (in joules) travelling from A to B in a car, than you do walking. And it's in the form of a crude oil derivative which is a "higher quality" of energy in relative terms too! More wasted energy.

Now that's all a gross simplification of the physical systems involved (inertia, friction and about a thousand other factors come into play which would refine the numbers in the equations, but not alter the overall picture). And i wrote it as much to see whether i could drag all that A'Level Physics up from memory :-) as make any particular point.

Except to say that when i refer to you "lugging the car with you", i mean "you're expending the energy to move that hunk of metal along with yourself".

Mr. Cominos would be proud!
(he's my old Physics teacher, by the way)
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Re: erm.... (Moon Cat)
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