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Re: Pushing a car uphill



On a winter's day, when departing from a parking lot whose temperature
is 10 DegF or less, a crossply tire can take a mile or two to lose its
flat spot.

Radial tires are much better in that respect.
Transmission oil viscosity is particularly variable with temperature:
a 100 weight aviation oil looks like a lollypop on a stick when checked
in [not very] cold conditions.

Brian

At 10:45 10/25/01 -0500, you wrote:
Could it be that the answer to Herb's question and the uphill question might
be the same? With today's radial tires, a car develops 'flat' spots pretty
quickly when at rest. Letting the car roll back a bit, or start it moving
forwards and you are off the flat spot (which rebounds quickly). With the
incline, it is a little easier starting the car moving down rather than up
the incline for obvious reasons. Does that make any sense?

Rick

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Professor of Physics
Saint Mary's College
Notre Dame, IN 46556
rtarara@saintmarys.edu

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----- Original Message -----
From: "Herbert H Gottlieb" <herbgottlieb@JUNO.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Thursday, October 25, 2001 10:14 AM
Subject: Re: Pushing a car uphill


On Thu, 25 Oct 2001 08:18:52 -0400 Wolfgang Rueckner
<rueckner@FAS.HARVARD.EDU> writes:
Kinetic friction is not involved here -- the car is not skidding and
neither are the ball bearings in the wheels. Wolfgang

It has been our experience that it is more difficult to
start pushing a stalled car on a level surface than it is
to keep it rolling once the car has started moving.
Why is this true??

Herb

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



Herb Gottlieb from New York City
(Where it's nice to live but I wouldn't
want to be a tourist here)
herbgottlieb@juno.com


brian whatcott <inet@intellisys.net> Altus OK
Eureka!