Energy considerations in strings

1. Jan 16, 2010

aim1732

1. The problem statement, all variables and given/known data
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. Relevant 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.

Last edited: Jan 16, 2010
2. Jan 16, 2010

Pengwuino

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

3. Jan 16, 2010

aim1732

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.

4. Jan 17, 2010

Pengwuino

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