Re: Car acceleration

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

Use walking as perhaps an easier example.  To start walking forward, there
must be a forward directed force acting on you.  How do you achieve this?
You push in a backwards direction with your foot against the ground.
Normally, you foot does not slip.  Your foot pushes back against the ground,
the ground pushes forward on you.  Why doesn't your foot just slip backwards
(as it might right now outside my window), because of friction between the
ground and your foot.  Without the friction (of the static variety) you go
nowhere.

Rick

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

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----- Original Message -----
From: Herbert H Gottlieb
To: PHYS-L@lists.nau.edu
Sent: Monday, February 04, 2002 1:57 PM
Subject: Re: Car acceleration



On Mon, 04 Feb 2002 13:09:49 -0500 Scott Goelzer <sgoelzer@EARTHLINK.NET>
writes:
> Is this a valid demo?
>
> Take a toy pull-back car. Pull it back and let it go. Ask what made
> it go? Student responses vary (usually the spring made it go) but seldom
> mention the table. Pull back the car, lock the drive wheels with fingers,
> release the car over the table in the air. Much spinning and noise, but no
> forward motion until contact with the table. The table provides the
forward force that makes the car go.
> Place a file folder on a bed of straws so that it slides freely.
> Place wound-up car on file folder and release. Car pushes road backwards.
> I've been using this for years as a third law/friction demo.




Newton's third law says that for every action, there is an equal and
opposite reaction. The usual example is a rocket ship in space with the
rocket exhaust
providing the "action" and the thrust of the rocket as the "reaction.
Please
explain (in simple language, if possible)  the part that friction plays