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

[Richard Tarara <rtarara@saintmarys.edu>]

If inertia is the tendency of a mass to resist a CHANGE in motion and 
forces are what cause changes in motion then the string can provide 
enough force to change the motion of the mass as long as that change is 
not too large...as long as the acceleration is not too large.  However, 
if we try for a large (quick) change in motion and therefore a large 
acceleration, then the string cannot provide that amount of force.  That 
is enough--no need to go into details of how the string breaks.  Has 
worked fairly well with students in my experience.  Of course I start 
out with a similar demo that shows students that they have a firmly 
ingrained instinct for all of this by using a roll of toilet paper on a 
rod.  Pull slowly to unroll, pull quickly to break off a piece.  At 
least it gets a laugh or two.

The ping pong ball and bowling ball can show the difference between the 
inertia of two objects (can use the two for momentum ideas as well).  
The mass and string extends the idea more clearly to the idea of 
acceleration...that the rate of change matters...something just moving 
the two balls doesn't.  My twist on this has been to get a volunteer to 
come up and place a hand palm up flat on the table.  I then drop a small 
rubber ball into their hand from about a foot above.  THEN...hover the 
bowling ball in the same position!  Both balls will be moving at the 
same speed when they hit the hand...so what's the big deal!   [Of course 
I never dropped the bowling ball...well not on their hands anyway.]

rwt

On 8/17/2016 2:22 PM, stefan jeglinski wrote:
> I think this is largely my objection to the tablecloth experiment. 
> While both this example and the tablecloth can be discussed in the 
> context of inertia, I don't think there's much there for the student – 
> what are your actual experiences with students? If I was a student and 
> was told this was a "demonstration of inertia and what it means," I'd 
> feel underwhelmed. All of the questions about fast and slow and what 
> causes the string to actually break are still there and unanswered. As 
> a student I'd say "the bowling ball has the same inertia in either 
> case - how is that the determining factor?" To the skeptical student, 
> I'd argue that you'd quickly leave appeals to inertia behind and talk 
> about acceleration and stick/slip friction (in the case of the table 
> cloth) and acceleration and tensile strength in this example.
>
> Again, I'm not saying that inertia has no place in this discussion – 
> it certainly does, esp as a starting point. but I feel like a better 
> /demonstration/ of inertia alone is closer to "here, try to move this 
> bowling ball and this ping pong ball and tell me what you think."
>
> [see next response]
>
> Stefan Jeglinski
>
>
> On 8/17/16 12:20 PM, 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....  
> > [Be sure to test that the string will break, especially if you wrap 
> > the string around your hand before the jerk.  ;-]
> >
> > rwt
> >
> >
> >
> >
> > O

--
Richard Tarara
Professor Emeritus
Saint Mary's College

free Physics educational software
http://sites.saintmarys.edu/~rtarara/software.html