Energy considerations in strings

[aim1732]

1. Homework Statement
The problem concerns a block(mass M) hanging from a light string attached to a fixed support, that is pulled down with a force of the block's weight that produces an extension of 'l'.


2. Homework Equations

The elastic P.E is half of (M*g*l).However the loss in gravitational P.E is (M*g*l), The question is where does the other half go?

3. The Attempt at a Solution
My guess is hysterisis as in rubber.

 

[Pengwuino]

The elastic P.E is half of (M*g*l).

Where did you get this from? This is not the potential energy for a string that obeys hooke's law.

 

[aim1732]

Where did you get this from? This is not the potential energy for a string that obeys hooke's law.

The p.e is YAl^2/2L. However F=YAl/L. Here the maximum tension here is M*g as the block is supposed to hang in eqilibrium. Hence half of M*g*l. Most textbooks abbreviate it as half of max.tension multiplied into extension.

As for the info it is from The Concepts Of Physics.

 

[Pengwuino]

Ah my mistake, I am quite rusty on this sort of stuff unfortunately. Hopefully someone else can help you here.