I am firmly in the camp of accepting the common meaning of "caused" when I decide to push on a puck on an air hockey table and it accelerates. I am comfortable saying I was the "cause".
However, when it comes to interpreting F=ma, there is certainly a problem with arguing causality because the force and acceleration arise simultaneously. When I decide to accelerate the puck, I have to move my finger to the puck in a deliberate action. During this motion, there is no "force" since my finger is not touching anything (neglecting air). It is only when my finger makes contact with the puck that a force and acceleration appear - and they do so simultaneously. I am responsible for a change in the state of motion of the puck, and I can control the amount of force and acceleration that appears, but the force of and on my finger was not the cause of the acceleration - it simply appeared along with the acceleration when my finger contacted the puck. My finger was no more the cause of the acceleration than when the puck hits a barrier and is turned around - the force of the barrier and the acceleration come about together.
Likewise, the stress and strain come about together. One cannot exist without the simultaneous existence of the other.
Bob at PC
________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of D.V.N. Sarma [dvnsarma@gmail.com]
Sent: Saturday, December 04, 2010 12:54 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] The "why" questions
Let us consider a body subject to two opposing forces
1. If the two forces are equal the net force is zero. But the condition
of the body is not the same as that of body on which no forces at all
are acting.
The body will be strained.
2 If the forces are not equal there will be a net unbalanced force
and there will be acceleration. But the condition of the body is not the
same as that of the body under the action of a single force of the same
magnitude. The strains in both the cases will be different.
It is incomprehensible to me why we object to the cause effect relation
between force and acceleration when we have no objection to the cause
and effect relationship between force and strain.
Or do we deny the cause and effect relationship between force and strain?