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.


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.

Andy