Elastic string. Ball. Kinetic energy. Confusion.

[swong1]

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!

 

[grzz]

At 0.5m above P there is no elastic PE.

 

[swong1]

Thank you for replying!

How come there is no EPE? Isn't some needed to get it back to its original shape?

 

[grzz]

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.