Sunday, February 21, 2010

Where my data originated

I'm getting some feedback on some of my physics, and statements about energy used by vehicles to accelerate, etc.

My source for my data is a book by Robert Q Riley, "Alternative Cars in the 21st Century". It was first published in 1994, so some of the data cited is research done in the 1980s, or early 1990s. Riley has put out a second edition of his book, and he has a website as well. In his first edition, he writes (page 106) "Aerodynamic drag is a product of the vehicle's frontal area, its drag coefficient, and the cube of its speed. The cubic increase of drag in relation to speed makes aerodynamics a major consideration when designing high-speed highway vehicles."

In my windpower research, I have also come across the "Vee-cube-d" power of wind, regarding wind energy and wind speed.


giordano bruno said...

Ivan Illich, a radical Jesuit writer of the 1960-70s had a concept called "convivial society" - roughly, a society where people could live together. He advocated 30mph as the speed above which people got less, overall living value.
There is a movement (name?) to calm suburban streets... when a fool drives fast past your house, dont shake your fist and scowl... smile and wave... the driver will suffer a common human reaction ... he will think you recognise him.. he may well slow down.
where I live congestion limits speed to around 25mph, which is just as well, since the motor-trikes are home made steel and would be lethal in a fast crash.
If only the deisels and bikes didnt idle in the jams... bring on hybrids.

My Physics graduate nephew lacked common-sence understanding of 2nd law thermodynamics.. he thought that high acceleration wasted no more than low acceleration, if the velocity attained was equal... this from a Berkely U post-grad!!

Why stop at 50kph... I say 30kph would be enough, within city limits. Plastic bubblecars with small batteries for all.
Highways will be buses or LR.

King of the Road said...

Well... it's physics, the units MUST match in any valid equation. It's not a sufficient condition, but it's absolutely necessary for a correct functional relationship.

Since drag is a force, the right side of any equation specifying must reduce to the units of a force (mass*length/time^2). The relevant physical quantities are air density, vehicle flat plate area, and vehicle speed if we assume an inviscid, incompressible (i.e., Newtonian) flow. These cannot combine to be a force with speed cubed. Power, on the other hand, must use speed cubed.

A (very basic) physical analysis of the momentum change of air flowing against a flat plate will also lead to the conclusion that the drag force is proportional to the square of velocity. As you've stated, however, power in wind and power required to overcome drag is proportional to the cube of speed.

cinzia said...

is it possible to email with you? i am a landscape architect very interested in a physical world at 35 mph and would love to be able to talk with you more on this subject?