Let's consider a gentle situation. A small puck is attached to a string. The string is sufficiently long in length. The setup is placed on a frictionless surface. One end of the string is nailed to the surface. Initially it was at a slackened state. Now we give the puck some velocity. The string end attached to it starts moving as well. The question arises, if at the slackened state there is no tension in the string, what causes the free end to move?
My thinking- Well, I think that the moment we gave puck an impulse to provide it with the velocity, some impulsive tension could have developed in the string in the length nearby to the puck. This impulse tension could have given the string's free end the velocity.
Then my subsequent question becomes, if there's no tension in the string in the slackened state, what causes the farther parts of the string (which were at rest initially) to start moving in the direction of puck's motion after a considerable time has passed? (and yes, I don't mean the parts nearby to the pinned end, but yes, the part far away from the free end.)
Note that the string is taken sufficiently long for this experiment.
After thinking a bit more about the problem, I am now certain that the impulsive tension could not develop in the string like I thought before. The string in the situation is taken ideal and slackened. Also, there is no friction or any other external force on the string. Nothing is pulling the string back so it won't have the tendency to "constrain" the puck's motion like a tight string would do.
Two of the three answers I've yet recieved try to imply (directly or indirectly) that even in the slackened state, the string would have some tension. Which would be growing steadily and uniformly. This, according to me, is incoherent with the general notion of slackened state of string meaning absence of tension. Also, this would not go along with the impulsive nature of the tension which occurs when there is a sudden development of tensile force.
I, hence, can't help but disagree with these views on the situation. If anyone has any different view or a clearer justification of the previous answers, you're always welcomed here.
- It would also be a great help if someone could explain what are the cause of those "constraints" as per @Bob D's answer.