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

That has never been a issue. If a ball bounces off a wall I think everyone agrees that there must have been a force on the ball during the acceleration. F=ma does not imply nor require cause and effect, either left to right or right to left. But it does not exclude it either - it's just irrelevant to the existence of the law.

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker
Sent: Monday, November 29, 2010 12:05 PM
To: Forum for Physics Educators
Subject: 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.
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