Re: [Phys-l] horsepower

[Brian Whatcott <betwys1@sbcglobal.net>]

At 01:35 PM 11/15/2007   Anthony Lapinski,  you wrote:

> For an "average-size" car, is there a general rule of thumb or formula
> that relates horsepower and speed? I read that it takes 20 hp to drive at
> 60 mph. Another source said 40 hp is needed to go 80 mph. I realize air
> resistance increases with the square of the speed, so those numbers seem
> to make sense.

Your numbers and scaling don't sound unreasonable to me either.
We could model the car's power requirement as a part that scales
with speed times the rolling resistance  of the tires: see
for instance  the examples at How Stuff Works:
    <http://auto.howstuffworks.com/tire4.htm>

   ...and a part that scales with air drag times speed, which
is proportional to speed cubed

If we suppose that the rolling resistance given at the URL above
is reasonable i.e. accounts for 12 HP at 75 MPH ( 12.8 HP at 80 mph)
then the air drag part would be accounting for 27.2 HP at 80 MPH
for a total of 40 HP.

For these presumptions, at 60 MPH, the rolling resistance accounts for
(60/80) of 12.8 HP or 9.6 HP and the air drag accounts for (60/80)^3
of 27.2 HP   or 11.5 HP, a grand total of 21.1 HP compared with your
 20 HP number



Brian Whatcott    Altus OK    Eureka!