From: ram@zedat.fu-berlin.de   
      
   Luigi Fortunati  writes:   
   >What is the difference between the force accelerating the mass (F=ma)   
   >and the force deforming the mass (Hooke)?   
      
     It's the same force, but applied to two different systems.   
      
     One system is a point of mass "m" that is not held in position.   
     When we call "F" the force acting on it, then the acceleration   
     of the point is "a".   
      
     When applying one version of Hooke's law, the force F is now   
     applied to one end of an elastic wire the other end of which   
     is fixed, and then the relative elongation e is proportional   
     to the force F (if the force if not too strong.).   
      
     So, when applied to one system, the force is proportional   
     to the acceleration, when applied to another system,   
     to the elongation.   
      
   >Can a force accelerate mass without deforming it?   
      
     The two laws just discussed had a simplified concept of a force:   
     they talked about a force applied at just one point: at a point   
     mass or at one end of a wire. When we continue to use this simplified   
     concept we can now apply a force to the end of an elastic wire which   
     is not held in position.   
      
     In this case, there should be some kind of deformation as   
     the parts of the wire near to the point upon which the force   
     acts start to move earlier than the remote parts of the   
     wire, when one imagines the wire as a grid of points   
     (atoms) connected by small springs (van der Waals forces).   
      
     Here both aspects of the force come into play: the points   
     have inertia (F=ma) and they are connected by springs which   
     deform (Hooke's law) when a force is applied. A mass point   
     at the end of an elastic wire can act somewhat like an   
     anchoring of the wire because of its (the point's) inertia.   
      
     For simplification, one might think of a one-dimensional   
     model: a long chain of links each of which is a mass point   
     and a spring. O////O////O////O////O////O///O////O////O.   
     Then, when one starts to push or pull the mass point at one   
     end of the chain, the effects start to travel through the   
     chain.   
      
     But when you have a small and rigid body and a small force, you   
     can accelerate the body without deforming it in good approximation.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)