Re: [Phys-l] the first law of motion

[John Denker <jsd@av8n.com>]

On 11/01/2011 09:53 AM, Anthony Lapinski wrote:
> The air resistance (and some internal friction on the wheels) will
> eventually slow the car down -- or else it would keep moving (and we know
> this doesn't happen). Outer space has no essentially friction, but ice
> does. So this is why I originally stated that static friction (ground on
> tires) is need to get a car moving. Maybe I am missing something here.

Well, let's talk about that.

> this is why I originally stated that static friction (ground on
> tires) is need to get a car moving.

That wasn't previously stated (let alone originally stated) in
the recent discussion.  That would not have drawn an objection.
We agree that the second law of motion says that a force is 
needed to /get/ the car moving.

Let's be clear:  The objectionable statements had to do with a 
force to /keep/ the car moving.  That's quite a different idea.

====

Here's another distinction:  
  a) If you want to talk about air resistance, that's 100% OK.
   That's physics.  We know how to quantify that.
  b) If you talk about some "force to keep the car moving"
   that is not OK.  That's not physics.  We cannot even begin
   to quantify that.

Specifically:  As soon as we formulate the problem in terms of
air resistance, we realize that if there's a tailwind there will
be less air resistance, and if the tailwind is strong enough the
car will actually speed up.  Formulating the problem this way leads
to interesting and useful predictions.

In contrast, you really really don't want to talk about some
vague force required to "keep" the car from coming to rest, as
if rest were the natural baseline state.  This is unquantifiable.
It violates the first law of motion.  It violates Galileo's
principle of relativity.  Et cetera.  It is a Bad Idea in so
many ways that I can't even count them.

To summarize:
 ++ Aerodynamic drag:  OK.
 ++ Internal friction in the tires:  OK.
 ++ Thrust:  OK
 ++ Balance of forces (thrust minus drag):  OK
 ++ Net force to /get/ the car moving:  OK.
 -- Force to "keep" the car moving.  Bad Idea.
  

If you draw a FBD and there are no forces in the direction of
motion, the body will keep moving!  No force is required to
"keep" the body in motion.

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