Deputy Dumbya Dawg wrote:
> "Bill Baka" <[email protected]> wrote in message
>
> : It would probably be easy to
> get stretch limo up
> : to 35 MPG as long as it stayed at 65 MPH.
I don't know about THAT. You might be able to *design* one that does so,
but you'd probably have to sacrifice a lot of things that are desirable
in a stretch limo. Perhaps engine power, perhaps interior space (to
allow for improved aerodynamics) ... just pulling things off the top of
my head.
> I may agree with you if this limo was in space but here
> on earth with gravity your argument does not hold
> water. The more weight (mass affected by the force of
> gravity) the more friction and the more energy to
> maintain the velocity.
No. You're neglecting inertia. A moving object tends to keep moving, and
if it's heavy it's harder to stop than if it's light.
> If you have your lime at 65mph
> and you stop putting energy into it, it will slow down
> and stop.
Yes.
> The more mass in the limo the faster it
> stops.
No. Let's ignore aerodynamic drag for a moment and pretend that the only
force slowing the car down is rolling resistance. The rolling resistance
of a car tire on asphalt is about 3% of the car's weight. Thus, the
deceleration force on a 1000kg car is 300N, and the deceleration force
on a 2000kg car is 600N. By Newton's second law, the first car
decelerates at a rate of 300N / 1000kg = 0.3 m/s^2, and the second
decelerates at a rate of 600N / 2000kg = 0.3 m/s^2. In other words, both
cars slow at the same rate.
Rolling resistance does increase with velocity, but on two identical
vehicles it will increase by the same amount for each, so the result
will be the same -- both vehicles will slow at the same rate.
Now it *is* true that the more weight you put on a tire, the larger its
contact patch with the ground. This *might* increase the coefficient of
rolling resistance, but only very slightly if at all (the material the
tire is made from and the surface it's rolling on have far greater
influence on rolling resistance). Let's say it's 3.1% for the heavier
car instead of 3% -- almost certainly an overestimate -- which would
produce a force of 620N. Over the course of a mile, that would require
32,000J of extra energy compared to the lighter car to maintain constant
speed.
Let's say the lighter car gets 30 mpg and both cars transfer energy from
the gasoline to the road at 25% efficiency. There are 120 million joules
in a gallon of unleaded gasoline, so 40 million joules are burnt each
mile. The extra 32,000J the heavier car requires each mile correspond to
128,000J/mile of extra gasoline. Therefore, the extra weight degrades
the car's mileage to 29.9 mpg. The difference of 0.1 mpg may as well be
zero considering that it's an overestimate to begin with, and that other
factors such as driving habits and regular vehicle maintenance (or lack
thereof) make a far greater difference in mileage than that.
Now if you factor in aerodynamic drag, both vehicles -- being identical
aside from weight -- will face the same drag force. They will expend the
same amount of energy overcoming it to maintain a constant speed.
However, if you let your foot off the gas and coast to a stop, you'll
find the heavier car coasts farther. I refer you back to Netwon's second
law to understand why.
