Re: Car acceleration

[Robert Cohen <Robert.Cohen@PO-BOX.ESU.EDU>]

My students would say "OK - the table is needed to make the car go; but,
just the same, without the spring, the car wouldn't go either"

I think asking what "makes" the car go is ambiguous.  Asking what "provides"
the force that makes the car go is less ambiguous but still has the problem
that we need to know the answer beforehand in order to know the intent of
the question.  Perhaps that is the purpose of the question?  Is there a
better way of phrasing the question that gets around all ambiguities?  Could
we just as well ask:

What forces are exerted on the car when the car is speeding up?  Which, if
any, are in the same direction as the car's acceleration?

Internal forces don't count since forces internal to the car are not said to
be exerted "on the car".
____________________________________________
Robert Cohen; rcohen@po-box.esu.edu; http://www.esu.edu/~bbq
Physics, East Stroudsburg Univ, E. Stroudsburg, PA 18301

> -----Original Message-----
> From: Scott Goelzer [mailto:sgoelzer@EARTHLINK.NET]
> Sent: Monday, February 04, 2002 1:10 PM
> To: PHYS-L@lists.nau.edu
> Subject: Re: Car acceleration
>
>
> 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.