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Re: [Phys-l] The "why" questions

The underlying physics is a construct. The asymmetry is not in my imagination, but in a practical evaluation of real-world events. If you want to say that my evaluation of those real world events is in my imagination, so be it. But then everything is in my imagination. I consider myself a radical constructivist (everything is basically a construct) in philosophy, but I don't apply that philosophy constantly in everyday life and in applying physics concepts. If I get hit by a bus, I'm pretty comfortable with the interpretation that the force exerted by the bus caused the acceleration of my body that resulted in my injuries. People understand abstract concepts by linking them to concrete events, and in applying a practical interpretation we can help students better understand the underlying physics.

On that note, we should return to Bob's purpose in starting this conversation. If stating that forces cause accelerations results in a confusion for students regarding the difference between velocity and acceleration, then it's a legitimate concern. But is it a major pedagogical concern if that isn't the case? There are pedagogical reasons for using the cause-effect argument, one of the most common being that students commonly add a centripetal force in circular motion that is caused by the centripetal acceleration. Personally, I have found that stating that forces cause accelerations, not the other way around, helps the students see their error. When we set up second law problems, we begin with free-body diagrams, regardless of the quantity we are looking for. This process comes first, and it seems a harmless thing to explain that we begin with the free-body diagram because the applied forces are what set up the application of F=ma.


While we might have a discussion here, largely a philosophical one, regarding the issue, I don't see why we should necessarily incorporate our philosophical concerns in all instruction. For more advanced students, yes. In more advanced applications, yes. There have been discussions here regarding the ability of students to reason formally, so is a practical, reality-based presentation of the second law such a bad thing?

I'm not saying we should avoid philosophical arguments altogether. It is important that students understand we are applying abstract models to the real world. But do we help students in general if we make a point of telling them that these forces don't cause accelerations, despite what is a reasonable interpretation of events?

Bill



On Nov 29, 2010, at 5:14 PM, John Denker wrote:

On 11/29/2010 03:01 PM, William Robertson wrote:

[1]
Here I'm making a distinction between a certain level of reality and
constructs from that reality. I stipulate that this argument falls
apart if you look at all of these quantities as constructs that we
place on the physical world.

On that we agree.

I believe there is a big difference between replacing a vector with
all sorts of combinations of component vectors, and working backwards
from a net force to the actual forces that make it up. You can
physically determine the magnitude and direction of applied forces
that make up the net force--not components along a set of axes but the
actual forces in a physical situation. You can also physically
determine the magnitude and direction of the acceleration of an
object. You can mathematically talk about component accelerations that
are due to individual forces, but you cannot measure them physically.
You can only measure the one acceleration of the object. There is an
asymmetry in the physical forces acting on an object and the physical
acceleration of the object.

See item [1] above.

If you can imagine a superposition of forces but not a superposition
of accelerations, that is a failure of your model. You have discovered
an asymmetry in your imagination -- not in the underlying physics.

Compound accelerations occur all the time in the real world.
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