# Tension in cables help please!

1. Nov 14, 2007

### elle

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

Hi, I'm actually teaching myself some basic mechanics/physics at the moment and I'm having difficulty getting my head around all the ideas.

Basically I've been reading up on a cables problem where a flexible, uniform cable of length L has its two ends held at a distance 'd' apart at the same height/level.

There is a corresponding graph which shows the relation between the tension and the length. The tension is large when L is small, and starts decreasing and attaining a minimum before increasing and becoming large again as L gets bigger.

Can anyone explain to me why this is so? Many thanks!!!

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

2. Relevant equations

3. The attempt at a solution

2. Nov 14, 2007

### Supernats

3. Nov 14, 2007

### elle

Hi, this was in a book, sorry!

4. Nov 14, 2007

### Supernats

Is 'd' constant?

5. Nov 14, 2007

### elle

Yes, the distance is constant

6. Nov 14, 2007

### Supernats

Well, if you're treating the cable like it has mass, I think the tension would be coming from the increasing force of gravity.

EDIT: Also, are we speaking merely of magnitude here?

Last edited: Nov 14, 2007
7. Nov 14, 2007

### elle

hmm i think so, im not too sure actually.

But why is tension large when the length is L = 1? and then it decreases? I can understand that if the cable gets longer, then there is more force needed in holding it up.

8. Nov 14, 2007

### Supernats

Because initially the cable is taught, that is, when L = d. When L increases just a little, T shoots down very fast because the length can't really "feel" the force. It's like if you're holding a length of string tight between two hands and then you move them in just a little bit, you feel the tension in the drop off incredibly.

9. Nov 14, 2007

### elle

Oh I see! that makes sense now

that's brilliant thank you! Thanks for your help

10. Nov 14, 2007

Of course.