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



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.