# Would the other end move instantaenously

1. Aug 18, 2010

### kevevans

It Light takes about 8 mins to travel the 93 million miles between the sun and the earth. If there was an iron bar in space along this distance and one end was pulled towards you would the other end move instantaenously ?

2. Aug 18, 2010

### nnnm4

The the disturbance would propagate at the speed of sound. You must remember in relativistic problems there are no perfectly rigid objects.

3. Aug 18, 2010

### Curl

No, it will be much much slower than 8 minutes.

Probably a few hours/days/weeks

4. Aug 18, 2010

### Staff: Mentor

....about 11 months by my calculation.

5. Aug 19, 2010

### Sakha

May I ask what equation you use to calculate that?

6. Aug 19, 2010

### cjl

The speed of sound in iron, which is around 5 km/s.

7. Aug 19, 2010

### Staff: Mentor

time = distance/speed

8. Aug 19, 2010

### n.karthick

Oh my doubt is why it takes months for other end to move? Since iron bar is rigid object it should move instantaneously right?

9. Aug 19, 2010

### AJ Bentley

Wrong.
Nothing is rigid.
All materials consist of atoms which are held together by nothing more than electric fields.

Movement of one end of a metal bar results in a wave of compression - just exactly the same as a sound wave in air (in fact it IS a sound wave), which moves down the length of the bar at the speed of sound for the material.

10. Aug 19, 2010

### Staff: Mentor

It is more accurate to describe the iron bar as a very stiff spring.

11. Aug 19, 2010

### n.karthick

Sorry I dont know anything about this. Could you please explain why it travels at the speed of sound? I could recall that speed of sound is due and depends on the properties of the medium at which it is travelling.
Here, coming to our iron rod example, though it is not rigid and it is like a spring or something like that, what makes it to travel at speed of sound in vacuum (no medium is present)?
Could you specify some references so that I can read and understand pertaining to this.

12. Aug 19, 2010

### AJ Bentley

Well, it's a tautology really.

Sound is just vibration. The speed of sound is the speed at which vibrations travel.
Vibrations travel at the speed of sound. It's the same statement.

Of course there's a medium - that's the bar.

13. Aug 19, 2010

### Staff: Mentor

He didn't say it travels at the speed of sound in vacuum. That wouldn't make any sense because sound doesn't travel at all in vacuum.

14. Aug 19, 2010

### Tomsk

Can anyone explain why the effect propagates at the speed of sound in the bar, and not the speed of light? The interactions between adjacent atoms is electromagnetic, so I would have thought it would be closer to c.

You can imagine giving the bar a very strong tug so that the end near you is moving faster than sound. Would that break the bar at some distance along it? Where? Could you accelerate it gently enough so that it doesn't break?

15. Aug 19, 2010

### n.karthick

Ok, I know sound requires medium to travel but I am curious to know how the compression wave in metal bar travels at speed of sound, that too in vacuum. I mean why it travels at such low speed. Why cant it travel faster than that? What is the theory/ mathematical derivation which shows that it travels at speed of sound. I am asking this, just for the sake of knowing the physics behind it. If you could suggest some material or book it will be helpful to me.

16. Aug 19, 2010

### AJ Bentley

Although the actual force travels between atoms at c, it's weakened by their separation and the effects of inertia mean there is a delay between the movements of adjacent atoms. It's exactly analogous with masses connected by (weak) springs.

17. Aug 19, 2010

### cjl

Quite simply, because that's the definition of the sound speed.

The speed of sound in any material is the speed of a pressure wave in that material (specifically, the speed of a pressure wave in the limit as the amplitude goes to zero - a large amplitude pulse would travel a bit faster). There's no real derivation, since that's just the definition of the sound speed.

18. Aug 19, 2010

### Danger

Something mentioned earlier brought a question to mind.
If the aforementioned iron rod (or rope, or cable, whatever), of any given dimensions, is perfectly homogenous, with no difference in molecular structure or cross-sectional area anywhere along its length, is there a way to determine where it will fail under excessive tension? For instance, if two bulldozers play tug-of-war with it, will it break in the middle, closer to one of them, or is it unknowable in advance?
I have no particular need to know this; it's just curiosity.

19. Aug 19, 2010

### kevevans

Thanks for all the replies. I never thought that the other end would move instantaenously. However, I am surprised that so many posts suggest that it is linked to the speed of sound. If this is so does it mean that no mechanical linkage can operate faster than the speed of sound. This does not sound (excuse the pun) correct??

20. Aug 19, 2010

### Danger

Right. Keep in mind, though, that for any "normal" scale, sound is very fast.