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