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



On 11/29/2010 09:43 AM, LaMontagne, Bob wrote:
A physics instructor places a toy car on a table and pushes it from behind. He/she then asks the class "What just happened?"

Student: The car just accelrated because you pushed it.

Teacher: We don't use "because" when we talk about force and acceleration.

Student: But I just saw you push it!

Teacher: Who are you going to believe, me or your own lying eyes?

jsd: How do you know I subjected the car to a definite force?
What if I tell you I subjected it to a definite acceleration,
such that you can infer the force from the acceleration and
not vice versa?

As a specific practical example, consider the acceleration
of a small tube riding in a massive centrifuge that spins
at a known rate. Do you really want to infer the acceleration
from the force? Wouldn't you rather infer the force from
the known acceleration?

This is not a metaphysical distinction. There are direct practical
engineering considerations involved. There are some things such as
DC torque motors that put out a definite force to an excellent
approximation. There are other things such as stepper motors that
allow me to dictate the position, velocity, and acceleration of the
load, independent of the force required, to an excellent approximation,
over a wide range of conditions. Once upon a time I earned my living
designing things that depended on stepper motors, so don't try to
tell me this isn't real.