physics question, energy related!?

Question

say that a string with a stopper at the end is tied down to an opening and then the stopper blown vertically off the opening at 5 m/s. How can i calculate the force acting on the string when the string is fully extended into the air? I know the mass of the stopper is 0.012g, the string is 0.46 m long, and i have other information which i'm not sure is useful (f.x. the inital force acting on the stopper as it blows off the opening is 64 N) Can someone please show me this calculation? Assume no air

Answer 1

This seems to be a rather intricate question. Are you taking advanced college physics?

When you say that the string is tied down, does that mean it is attached at the stopper's initial level? And when you way fully extended, do you mean that the stopper has reached a height of 0.46 meters and the string is restraining it from rising any higher?
If that's the case, then the string must INSTANTANEOUSLY reduce the stopper's velocity to zero from 5 m/s, less the deceleration already caused by gravity. (There would also have been some deceleration caused when part of the stopper's initial momentum was used in order to accelerate part of the string upward, but we are probably supposed to ignore the mass of the string.) And actually the stopper won't be stopped instantaneously, as the string will have some give to it (some elasticity). So, depending on how much elasticity the string has, there could be a very large force over a very short time (not much elasticity), or a smaller force over a longer period of time.

As I say, this seems to be advanced physics (or basic engineering), requiring clever use of calculus, and requiring that you make a number of assumptions about the characteristics of the string.

Sorry I can't go further with the information provided here.

Answer 2

The initial force accelerates the stopper. It flies up, but it is being slowed by gravity. After 0.46 meters, the string stops it.

You'll need to calculate the speed of the stopper after it has travelled 0.46 meters against gravity. Then that speed plus its mass will give you its momentum, from which you can calculate the force on the string.

If you were in an advanced class, you might have been given the mass of the string! And might not be able to neglect that ... which also adds the the deceleration of the stopper. You'd need integration to solve that part of the problem!

The initial force acting on the stopper is thrown in there to confuse you. Teachers do that because in the real world you need to sort out what matters and what doesn't.