Re: Spring Problem

[Margaret Mary Mazzolini <304130@bud.cc.swin.edu.au>]

Good question Steven, this is one that I try and get my students to 
grapple with if they are really going to come to grips with Newton's Laws 
- and I learn something from my discussions with them, every time. (I do 
it with the old horse pulling the cart & the cart pulling back on the 
horse analogy.)
To get far with Newton's 2nd and 3rd laws, you have to be really careful 
about mentally isolating what the force(s) is/are being applied to - i.e. 
what is the "system", in Physics jargon.  If you want to know about what 
happens to the end part of the spring, then that is your system.  The 
only forces external to that are the force of you pulling on it, the 
force of the rest of the spring pulling back on it (-kx), (and gravity & 
the reaction from the table).  The other force you mentioned, of the 
spring pulling back on the hand, is just that - i.e. a force on your 
hand.  It affects the acceleration of your hand, but not the acceleration 
of the end of the spring.
Hope that helps
Cheers
Margaret Mazzolini 
  

On Wed, 29 Oct 1997 SSMOTHERMAN@MSCC.CC.TN.US wrote:

> 	I have a question that, on the surface, appears so simple, I'm
> embarrased to ask it.  But it's something that has me stumped.  Maybe there is
> something obvious that I'm overlooking.
> 	We have a horizontal, unstretched spring connected at one end by a
> rigid support.  I want to apply a force at the other end to stretch the spring. 
> By the 3rd law, the force I exert to the right (F=kx) will always be equal in
> magnitude to the force that the spring exerts back to the left (F=-kx).  In
> this case, how can there ever be an acceleration of the end of the spring?
>
> Steven Smotherman
> Motlow State CC
> Tullahoma, TN