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

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?

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