I am new to the site and donot have the reputation to comment, and hence I repost the question What is the force exerted by a spring when pulled by a force $F$? I was not satisfied with the answer there because my doubt is about the case where there is NO MASS attached to the spring. If there is some mass attached to the spring, we can analyze forces between the spring and the mass, and that between the mass and the other force pulling it (say the gravitational force pulling a mass downwards with the mass attached to a spring which in turn is attached to something rigid, say a ceiling.)In this case, one can, with some thinking understand how Newton's 3rd law holds. The force applied by the spring on the mass (and also that applied by the mass on the spring) will be equal to kx, and is independent of the force due to gravity on the mass (in the sense that the spring force is dependent only on its extension, which does depend on the force with which the mass, which pulls the spring is pulled, though)
However, If there is NO MASS attached to the ideal spring, how can I apply Newtons 3rd law? Say a force of 10 newtons is suddenly applied to the end of an unstretched ideal spring (Lets assume there is some mechanism to do so ).How will Newton's third law hold till the string gets stretched to some length?
Or, is it wrong to think of such a case? I mean, am I wrong in thinking of the spring as an object on which force can be applied? is it just a "source of force" and not an object on which force and the force laws can be applied?
Edit: To be more precise, will I be right in saying that a real spring can be thought of as an 'ideal spring + the mass it has', with its mass in a way playing the role of the object on which the ideal spring('the source of force' and not an object) exerts force? Applying force on the ideal spring is hence meaningless?
If it can be considered an object on which force can be applied, what will be its reaction to an external force ? will it infinitely stretch since its massless? ( and since we do not know the source of the external force as well...it can be a non-contact force. There are only two elements here...the ideal spring and the source of external force)
Could you please add clarity to my understanding of springs?