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Re: [Phys-l] [PTSOS] anvil and sledgehammer inertia demo

What can we really "prove" in our physics classes? We can never eliminate
friction, so a cart on an air track will eventually slow down. But it will
take a longer time to do that than if the air were turned off. That's why
I like using feathers and coffee filters.

We can't "turn off" gravity, but we can still talk about what would happen
if we could (object on table will not rise upward).

Or what would a bathroom scale read if we jumped off a building? We can't
do this, but we can suspend a block with a spring scale, let it go, and
see what happens.

Physics is PHUN!

Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
It seems to me that all 3 of Newton's laws involve this kind of chasing!
You can "demonstrate" these laws, but I don't think you can "prove" them
in any non-circular way.

For example, look at the slider on the air track. It goes for a long
while after an initial push, but it DOES stop eventually. A student
could reasonably conclude that Aristotle was right, but that with less
friction, it just takes longer to come to a stop. But even if you ignore
that slow deceleration, a skeptical student might suspect that the air
jets that lift the slider might also be providing a small sideways push,
thus keeping the slider from its natural state of rest!

Or here's another example. Even as I type, my students are doing the
experiment where you hang a mass over a pulley to pull a car along a
track. They will do one experiment where they move little masses from
the cart to the hanger, thus "proving" that acceleration is directly
proportional to force and another experiment where they add mass to the
cart, "proving" that acceleration is inversely proportional to mass.

I think this lab has great value, but I don't think it "proves" anything.
These are first year students and it is still September. I am happy if
this lab helps them to remember the direct and inverse proportions
involved in the second law. It is also their first experience collecting
data like this and then analyzing it with a spreadsheet. So I think it's
worth doing. But I know it is not a proof -- for example, in the second
experiment, I claim that the force on the system is constant because we
do not change the hanging mass. (I'm not claiming that the tension in
the string is constant, just the force on the "system".) But how do I
know that's true? Without using the second law, I can't show that force
to be constant.

What I think we can claim is that the three laws together make a
coherent system that we can then use to make predictions about the
behavior of objects. But to me, the laws are like axioms.

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Joseph Bellina
Sent: Thursday, September 30, 2010 7:48 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] [PTSOS] anvil and sledgehammer inertia demo

What is wrong with chasing your tail? Since we don't in principle
know what a force is, the only way we can say the forces are balanced
is to observe the motion.

joe

Joseph J. Bellina, Jr. Ph.D.
Emeritus Professor of Physics
Co-Director
Northern Indiana Science, Mathematics, and Engineering Collaborative
574-276-8294
inquirybellina@comcast.net




On Sep 29, 2010, at 11:12 PM, Derek McKenzie wrote:


To demonstrate equilibrium i.e. balance of forces, put
the puck "here" and "there" and observe that it remains
at rest. Then give it a push and observe uniform motion.

The vertical forces are balanced. The horizontal forces
are nearly zero.


Just out of interest, how do you justify the claim that the forces
are balanced, without chasing your tail?

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l