From: ram@zedat.fu-berlin.de   
      
   Luigi Fortunati  writes:   
   >Stefan Ram il 19/02/2023 16:01:21 ha scritto:   
   >I had never heard this one: does F=ma only apply to material points and   
   >not to bodies?   
      
     The point is a simplification. An extended body has a mass density   
     at each of its points, and the force has to be replaced by   
     a force field, which assigns a force to each point in space.   
      
   >This is precisely the profound meaning of my question: are they two   
   >different types of force, how different are acceleration and lengthening?   
      
     In "F=ma", "F" means "/the sum/ of all forces acting on the point".   
     When you pull one end of a spring, your pull is one force, but the   
     spring also exerts another force. When the end of the spring stops   
     moving, the sum of these two forces is zero, so, F=0 and a=0. The   
     lenght of the spring actually describes the force that the spring   
     exerts on its ends.   
      
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