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

[John Denker <jsd@av8n.com>]

On 08/16/2016 07:52 PM, stefan jeglinski wrote:
> Like I said, I do understand how one could use this demo to discuss 
> the first law, but it seems to me that a glider on an air track or a
>  puck on an air table are more instructive given a finite class time.
>  What does everyone think?

I reckon there is a tradeoff between the science content and
the pedagogical / motivational factors.  Both are important.
The tradeoff is nontrivial.

The advantage of the tablecloth demo is that students can relate
to it.  They could do it at home.  In contrast, they are not likely
to have an air track at home.

> he explains that the dishes remain on the table when you whisk a 
> tablecloth from under them "because of inertia."

IMHO that's not an explanation.  That's just a bunch of words.

> it seems that friction is one of the reasons that people don't really
> get the First Law: "objects in motion stay in motion" but virtually
> everything that you slide across a table doesn't do this. Isn't this
> regarded as one of the reasons that the force-and-motion connection
> became so ingrained?

Yes, it is.

Here's how I explain it to beginners:
  It's a question of what you consider "normal".  For thousands of
  years, people considered friction to be "normal", not requiring an
  explanation.  Then one fine day, Galileo decided that free motion
  was normal, and all forces, including frictional forces, require
  an explanation.  You can do it either way, but as we shall see over
  the course of the year, some things are treeeemendously easier to
  understand if we adopt Galileo's approach and account for /all/
  the forces.

  Example:  Pendulum.  You can set up a pendulum to have very little
    friction.

  Example:  Astronomy.  There is very little friction in outer space.

  Fact:  Ordinary frictional forces are not as strong as you might
    have guessed.  Often it takes some time for them to appreciably
    change the momentum of an object.

    Example:  Driving.  You have to apply the brakes well /before/
       you reach the stop sign.

    Example:  Tablecloth demo.  If I pull the cloth fast enough,
       it does not impart much momentum to the dishes.

       BTW:  If you do this at home, make sure the cloth does not
       have a hem!



===========================
Beginners don't need to hear about this (unless they ask), but here's
the next level of detail:

  Non-obvious fact:  The magnitude of the force of sliding friction
    is almost independent of speed over a rather wide range.  (This
    is very different from aerodynamic drag.)  So we have nothing to
    lose and much to gain by pulling the cloth quickly.