Re: [Phys-l] the importance of F causing ma (and not vice versa)

[Rick Tarara <rtarara@saintmarys.edu>]

----- Original Message ----- 
From: "John Denker" <jsd@av8n.com>


> Rick Tarara wrote:
>
> > That forces _cause_ accelerations then is a pretty important concept
>
> OK, I'll bite.
>
> Let's forget everything else and discuss this one point, namely the
> _importance_ of "knowing" that forces cause accelerations (and not
> vice versa).
>
> Suppose I'm a student in your class.  Please motivate me.  Please tell
> me why I need to "know" that forces cause accelerations.
>
> In particular, suppose I plan on having a research career, calculating
> forces, measuring forces, calculating accelerations, measuring 
> accelerations,
> et cetera, all day every day.
>
> What penalty will I pay for not "knowing" that forces cause accelerations?
>
> The question is addressed to everyone, not just Professor T:  What will I
> be able to do with "force causes ma" that I can't do with "force equals 
> ma"?

Well, if you are in my class you are probably a communications or econ 
major, maybe even an art major, or if you are in my other class you are a 
Chem major.  If you are like the most students (most people) you think that 
ALL motion IS caused by forces (pushes and pulls) and that anything moving 
has a force acting in the direction of motion (the direction of what we 
'Physics Pros' call velocity).   Now my main aim is to help you understand 
how science, in particular physics, has changed our understanding of the 
world around us.  To that end, I will endeavor to have you understand 
macroscopic motions from a Newtonian point of view.  To do that, you will 
need to abandon your Aristotelian pre-conceptions.  After working on a way 
to describe motions--a way that communicates what we are seeing in a way 
that everyone can be on the same page, we then will be ready to work on how 
to produce these motions.  Since you already have an instinct for forces 
(wrong as it might be) and an instinct for causation, one that will not be 
undone easily, we will work with that.  From here we will work on Newton's 
laws.  It will be difficult.  You will not want to give up your notion that 
a moving object has a force causing it to move.  You will have a tough time 
understanding acceleration--how it is different than speed or velocity, but 
if successful you will ultimately come to see that what is really 
interesting in any motion is change--in fact motion itself is change.  You 
will, again if you work hard enough at it, will come to understand that only 
if the velocity of an object actually changes do we need that 'causing' 
force.  Then, as it seemingly always is, things get complicated.  The book 
sitting on the table is not accelerating, yet there are clearly forces 
acting on it.  But that is OK--forces can be added, as long as we keep track 
of the direction of forces, and sometimes they add up to zero.  That is the 
case for the book, and you will understand that pretty well.  However, when 
a push a cart across the carpeted floor at constant speed and contend that 
again the forces on the cart add up to zero, you will have a tough time. 
The cart is moving!  Ah, but it is not accelerating.  If I take away my push 
and eliminate the friction with the floor and air, the cart continues to 
move at a constant velocity.  Tough, but maybe you get it.  Now if I want 
the cart to stop or to speed up, NOW I need a non-zero force (or set of 
forces) to act.

All goes well until I throw a ball straight up and ask you to fully describe 
the motion--velocity and acceleration, and then describe the forces acting. 
Well if you can get that, you WILL pass the course, and with a good grade.

So, why do you need to know any of this--to be moderately well educated.  To 
understand the world a bit better, even if only at a 17th Century level of 
understanding.  Why did we let you stay with a causation understanding of 
the relationship between acceleration and forces--because we only had you 
for a couple semesters and to wean you from ALL your pre-conceptions would 
take a bit more time, and besides, the professor thinks that forces cause 
accelerations.   ;-)


Rick

*********************************************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
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