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Re: [Phys-l] the first law of motion (was: FBD)



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.
With the banana peel example, I do understand that walking is different
from riding in a car.


Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
On 11/01/2011 08:15 AM, Anthony Lapinski wrote:
Also hard to drive
on ice, but if the car is moving it requires little force to keep moving
as ice has low friction coefficient.

Gaack!

Again I say: Talking about the "force to keep moving" is a
Bad Idea. It sets physics back almost 400 years.

If you want to argue the point, I'm willing to listen, but you
need to actually explain what you mean and why you think it
makes sense. If you just mention "force to keep moving"
without explaining it, there's going to be an objection every
time.

The Aristotelian model ("an object in motion tends to come to
rest") is 400 years out of date. It is a Bad Idea in practical
terms and a Bad Idea in pedagogical terms.

For one thing, let's contrast the low-friction case (ice) with
the extreme high-friction case. For a cog train, the coefficient
of friction is effectively infinite ... but this does not result
in any "force to keep moving".

The same goes for the ice, when the car is already moving: There
is, to first order, no "force to keep moving" that is reduced when
the moving car transitions from dry pavement to iced-over pavement.

Think about what the "F" means in "FBD".

There is no "force to keep moving" in any FBD.

If your FBD is detailed enough to include things like aerodynamic
drag, you may want to include enough trust to overcome the drag
... but neither of these should be described as a "force to keep
moving".


Again: The Aristotelian model ("an object in motion tends to come
to rest") is 400 years out of date. It is a Bad Idea in practical
terms and a Bad Idea in pedagogical terms.

The first law of motion says an object at rest tends to remain at
rest, and an object in motion tends to remain in motion. You
don't ever want to deviate from that or disguise that.

Consider for example the motion of the planets: They keep going
for billions of years with no appreciable tendency to come to
rest. Galileo was able to parlay knowledge of cosmology into
knowledge of terrestrial motion.

That's important, and it's also just plain nifty. The motion of
planets tells us something about the motion of cars. This illustrates
the power and the unity and the grandeur of physics.
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