Re: [Phys-l] racing

[brian whatcott <betwys1@sbcglobal.net>]

On 10/25/2011 1:15 PM, Anthony Lapinski wrote:
> I'm teaching about friction now and a discussion came up about race cars.
> I mentioned that these cars usually have a spoiler on the back end so that
> air pushes down on the fast car, which increases the apparent weight,
> which increases the frictional force. This is good as it makes the car
> accelerate more on straight track and more stable on turns
>
> Kids mentioned that race cars can "flip over" if they go too fast. My
> question is how does a race car flip over going fast on a long stretch of
> straight track.? Ignoring the spoiler, aren't these cars designed so that
> they will grip the road better due to the aerodynamics? Or do most cars
> become airborne due to interactions with other cars or air currents on the
> track?
>
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Things that are basically flat underneath and curved on top, or where 
the airspeed
runs much faster above than below, con develop lift.    If they were 
shaped like a
low drag wing section, the lift would tend to rotate the shape nose up, 
which is
 an unstable circumstance. You will recall that most wings proceed from
 zero lift to maximal lift in about 15 degrees of rotation - so it 
doesn't take
much of a pitch upset to trigger an instability. For a wedge-shaped object,
the variation of lift with pitch might proceed differently, and this is 
one way
 to think about surfaces intended to progressively push down harder with
 increasing speed.

Brian W