Andy Champ wrote:
On 09/09/2012 20:10, The Natural Philosopher wrote:
Andy Champ wrote:
On 09/09/2012 15:27, The Natural Philosopher wrote:
Whether you call that friction is up to you. Personally I dont.
Friction to me is sliding contact between hard surfaces, or fluid over
a surface.

If you did in fact have a classic frictional case as a boat hull in
water, you very quickly find that cars go faster than boats of the same
weight for the same power

That depends. At very low speed the friction of a floating object is
negligible; that isn't true of a car. I'll happily push a 10 tonne
boat off from a jetty, but I wouldn't even try with a truck.


Tyre rolling resistance represents almost a fixed drag component
irrespective of speed.


Boat hull resistance is purely a function of wetted area and speed.

I see you don't sail much.

Au contraire getting a boat to plane is about getting the wetted area
down


Nope. Getting planing is all about the wave systems. Once you are
planing things change; 10 tonnes being pushed off a jetty isn't planing.
Once planing the hull shape is also pretty important - try tying a
bucket to an outboard leg, and see how well a motor boat planes. And
think how small a proportion of the wetted area that is.


Air resistance is more complex, because air is more compressible.
Its a
function of frontal area and shape.

... and you won't notice the compressibility of air until you get to a
substantial fraction of the speed of sound.

I see you dont do physics or aeronautics. OR have a car with tyres that
are inflated.

OK, so just how much does the air compress around a car at 70MPH?

depends. Maybe a few PSI.

Ask McLaren.

Or an aircraft designer. Enough to create a pressure difference big
enough to keep a plane up.


The compressed air in the tyres isn't relevant to the air drag.

Andy


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