Elastic string. Ball. Kinetic energy. Confusion.

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Homework Help Overview

The problem involves an elastic string with a specified modulus of elasticity and natural length, attached to a ball of known mass. The scenario describes the ball being pulled down and the subsequent inquiry into its kinetic energy at a certain height above a fixed point.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to relate previous calculations of kinetic energy when the string was slack to the current problem. They express confusion regarding the extension of the string and its implications for elastic potential energy (EPE). Other participants question the presence of EPE at a specific height and discuss the relationship between extension and potential energy.

Discussion Status

The discussion is ongoing, with participants exploring the definitions and implications of elastic potential energy in the context of the problem. Some guidance has been offered regarding the conditions under which EPE is present, but no consensus has been reached on the implications for kinetic energy calculations.

Contextual Notes

Participants are navigating assumptions about the elastic string's behavior at different extensions and the relevance of prior calculations to the current scenario. The original poster's reference to previous work suggests a potential constraint in understanding the transition between states of energy.

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Homework Statement


An elastic string with modulus of elasticity=12. Natural length= 0.5m. Mass of ball attached to the end of the string=0.5kg. The other end of the string is fixed at point P. The particle is pulled down until it is 1.5m below P.

Find the kinetic energy of the particle when it is 0.5m above P.



The Attempt at a Solution


The previous question asked me to prove the KE when the string was slack and I got 7.1J, I'm not sure if that is of any relevance when working out this question.

I'm confused as to what the extention is when working out the EPE or if there even is one. Any help will be greatly appreciated!
 
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At 0.5m above P there is no elastic PE.
 
Thank you for replying!

How come there is no EPE? Isn't some needed to get it back to its original shape?
 
I am assuming that EPE means elastic potential energy. But EPE is directly proportional to the extension in the string. But since the natural length of the string is 0.5m, then at 0.5m above P the string is not extended and hence has no EPE. Of course the mass has KE and grav PE both of which are the result of the original EPE when at the lowest point.
 

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