Re: Pushing a car uphill

[Herbert H Gottlieb <herbgottlieb@JUNO.COM>]

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.

Herb  Gottlieb