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Re: [Phys-L] inertia and the tablecloth demo

On 08/17/2016 05:53 AM, Bruce McKay wrote:

Friction is an issue beyond Newton’s first law.

That's ambiguous. I would say that we need to talk about
force in conjunction with the first law /and/ beyond.

Our syllabus describes frcition as a force that opposes motion.

Where's That From?????

I ask how could we move if there was no friction?

Good question! The syllabus doesn't make any sense.

In the _Principia_ and ever sense, motion means momentum.
It's in the same direction as the velocity. The second
law tells us there is not the slightest reason to think
that the force is in the same direction as the momentum.

Perhaps the simplest counterexample to the syllabus is a
car driving in a circle at constant speed. The force is
not opposite to the motion. In fact it is perpendicular.

Similarly, a small block sitting on a turntable, held in
place by friction, also exhibits a force perpendicular to
the momentum.

A car driving up a slight incline depends on a frictional
force in the same direction as the motion.

Similarly, a block sitting on a conveyor belt, moving
relative to the lab frame, being carried up a slight
incline, exhibits a frictional force in the same direction
as the motion.

Most importantly: The principle of relativity (as set
forth by Galileo) is even more profound than the first
law of motion (also set forth by Galileo). It guarantees
that we can change the motion without changing the force,
just by choosing a different reference frame.


===========================
On 08/17/2016 09:20 AM, Richard Tarara wrote:

BTW: A really simple demo that deals with inertia and especially
emphasizes the 'change in motion' aspect of the physics is to simply
tie a string to a 1 kg mass. Pull upwards on the string and the mass
rises. Jerk hard on the string and the string breaks. Discuss....

Yes, that's a good one.

A fancier version is discussed here:
https://www.av8n.com/physics/ill-posed.htm#sec-snap-thread