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

[Richard Tarara <rtarara@saintmarys.edu>]

Not sure what your problem is!  One of the things I take from the third 
law is that forces are interactions and it takes two to tango.  If A 
acts on B then B acts on A.  A and B interact with each other.  The 
problem with Anthony's question is that to be in 'proper' third law form 
the 'weight' force needs to be expressed as the Earth pulls down on the 
book (yes this is Newtonian gravity not GR--not sure how to express it 
in GR).  If half of the interaction is a pull, then the other half must 
also be a pull and in this case the book pulls up on the earth.  That IS 
the hardest one for students unless you can get them to think about 
making the book larger and larger--say until it gets to be big enough to 
have the same mass of the earth.  Now separate them by some distance and 
let them go.  Will the book fall to the earth as it does when a normal 
book is released from a meter above the surface of the earth?  Well you 
of course know the rest.  In a purely practical and perhaps 
'engineering' way, I see the first law as showing us WHEN we need forces 
(either to balance other forces or to get something to accelerate.  [I 
know the purist don't want to say that a force can cause an acceleration 
but in everyday practice I really don't see the problem...especially 
when just trying to move people from an Aristotelian to a Newtonian 
world view.]  Anyway, then the second law gives you the prescription for 
determining how much force you will need to get your desired 
acceleration (which might be zero) or how much acceleration you will get 
with a certain set of applied forces.  Again in practical terms, the 
third law helps you to identify forces.  A prime example of this is to 
stand in front of a class (after dealing with the laws) and ask them to 
tell you exactly how to walk across the room.  Lot's to consider but 
when they identify that you need a net force acting on you in the 
direction of intended motion, that force is quite elusive UNLESS you 
apply the third law.

On 8/19/2016 6:56 PM, Derek McKenzie wrote:
> Anthony Lapinski wrote:
>
> As I stated before, my students (regular and honors) find forces to be
> the
> hardest topic. The third law is particularly challenging. A book rests
> on a
> table. If the book's weight (true weight, mg) is the action force, then
> the
> reaction force is the ___ on the ____ acting _____. Nobody gets this
> right!
>
> ---
>
> No offense, but I take the view that that can be a sign of a poor
> question.
>
> I'm not sure exactly what you're looking for myself to be honest.
>
> Can you tell us the answer you're hoping for?
>
> Derek McKenzie
> http://PhysicsFootnotes.com
> _______________________________________________
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--
Richard Tarara
Professor Emeritus
Saint Mary's College

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http://sites.saintmarys.edu/~rtarara/software.html