Elongation of a pendulum?

1. Apr 16, 2012

wotanub

Elongation of a pendulum???

Not really a specific question just something in general.

When discussing simple pendulums (a ball on the end of a string) or any system involving strings, my textbook always refers to something as the "elongation" of the string?

Before this, I had never heard of this! When you swing a pendulum, is the string longer at some times than others? How does that work? What causes this? What effect does it have on the motion of the pendulum?

I asked a professor today and he said something like a string can be considered as something like a spring that can be stretched, but not compressed, and there can be a spring constant associated with it. If this is so, then why doesn't this "elongation" factor show up in equations like ω = √(g/l) for the simple pendulum, and what does it mean for the normal modes of coupled pendulums? I was trying to ask him all these questions, but his English isn't so good to have a long discussion.

2. Apr 16, 2012

Nabeshin

Re: Elongation of a pendulum???

This is right. The reason it doesn't show up in these equations is because they're just that: simple pendulum equations. They assume the 'string' or 'rod' or whatever it is on the pendulum is a) massless and b) perfectly rigid. One could do an analysis assuming otherwise, but for a lot of physical systems these assumptions are pretty good.

3. Apr 17, 2012

wotanub

Re: Elongation of a pendulum???

In what case dies the elongation of a pendulum have a measurable effect on the dynamics of a system? You would think such a phenomena would have an effect on the equations of motion.