Re: Pushing a car uphill

[Mark Sylvester <msylvest@XNET.IT>]

At 22:59 24/10/01 -0400, Herbert H Gottlieb wrote:
> On Wed, 24 Oct 2001 21:02:11 -0400 Edwin K Seppa <ekseppa@JUNO.COM>
> writes:
> >
> > One of my students asked, "Why is it easier to push a car (by hand)
> > up a slight incline if you first let it roll back a bit?"
> >
>
> There are two types of friction, static and kinetic.
> When a car is stationary, a large force is required to
> overcome the static friction. Once it is rolling,
> the static friction disappears and a lower value
> of friction, called kinetic friction takes over.
>
> According to the current New York State Regents Reference
> tables, the coefficient of static friction between
> rubber and dry concrete is 0.90 but that of kinetic friction
> on between the same sub stances is only 0.68.

...which throws light on what happens when a car goes into a skid. In this
case of course we're interested in friction in the wheel bearings, which I
presume use ball bearings just like bicycle wheels. Ball bearings roll, so
does kinetic friction enter the picture at all?

Mark


Mark Sylvester
UWCAd
Duino Trieste Italy