Long rod question

moon man · 30 Jan 2009 at 20:18

im sure its something you all wonder about but if i have a metal rod 10 miles long and pull it at say 1 metre/second how long before the other end moves assuming its frictionless and wont stretch, ok its not instant or speed of light so how fast is that motion transfered

Dr.EM · 30 Jan 2009 at 20:19

At a complete guess, whatever the speed of sound is in the given metal?

Le_Petit_Lapin · 30 Jan 2009 at 20:22

If its frictionless and it doesn't stretch, then it moves at the same time as you pull the end you've got in your hand.

Unless this is like that plane on a treadmill question.

Ma'am · 30 Jan 2009 at 20:24

Does it matter seeing as no one is going to have a metal rod 10 miles long and even if they had would they be daft enough to try and pull it?
I've got a metal rod that's 4 feet long though and it doesn't take much pulling. Is that any help?

qwerty · 30 Jan 2009 at 20:24

Simultaneously

Le_Petit_Lapin · 30 Jan 2009 at 20:25

Ma'am said:
Does it matter seeing as no one is going to have a metal rod 10 miles long and even if they had would they be daft enough to try and pull it?

If I had a rod that was 10 miles long I'd wave it about and pull it all day long

neiljung · 30 Jan 2009 at 20:25

qwerty said:
Simultaneously

This.

The Mad Rapper · 30 Jan 2009 at 20:26

moon man said:
im sure its something you all wonder about but if i have a metal rod 10 miles long and pull it at say 1 metre/second how long before the other end moves assuming its frictionless and wont stretch, ok its not instant or speed of light so how fast is that motion transfered

It would be instant. Well, instant to you anyway.

moon man · 30 Jan 2009 at 20:27

instants impossible

mangobreeder · 30 Jan 2009 at 20:27

V = s0 / at

S = s0 + 1/2 (v0 + v)t

S = s0 + v0t + 1/2 at^2

V^2 = V0 ^2 + 2aD2

S = S0 + VT - 1/2at^"

where...

v is the body's initial velocity
s is the body's initial position

and its current state is described by:

v the velocity at the end of the interval
s the position at the end of the interval (displacement)
D the time interval between the initial and current states
a the constant acceleration, or in the case of bodies moving under the influence of gravity, g.

Dr.EM · 30 Jan 2009 at 20:27

Why is it assumed to be instantaneous at what is effectively a very low frequency (you pulling it) as compared to the speed at which it would transmit higher frequencies (sound waves)? Is it because it's below the "breakup" frequency of the material (term borrowed from speaker cones!)? Surely at that length this frequency would be extremely low anyhow.

Ma'am · 30 Jan 2009 at 20:28

Le_Petit_Lapin said:
If I had a rod that was 10 miles long I'd wave it about and pull it all day long

Rofl. Would it be waveable though? Think of the effort involved

Le_Petit_Lapin · 30 Jan 2009 at 20:28

moon man said:
instants impossible

Well fine, at the speed of light, which would be pretty bloody quickly if its only 10 mile long.

Ma'am said:
Rofl. Would it be waveable though? Think of the effort involved

What's the point in a 10 mile long rod if you can't wave it about?

Felix Da Housecat · 30 Jan 2009 at 20:29

It would be the same as pulling a 1 meter metal pole, the ends move at the same time.

If it's solid then it wouldn't matter how long it is, it's still going to move at the same time.

penski · 30 Jan 2009 at 20:29

Depends if you're pulling toward or away from the greenwich meridian...It could 'move' before you pull it, based on local time...If the pole's long enough.

*n

Jonny69 · 30 Jan 2009 at 20:29

Le_Petit_Lapin said:
If its frictionless and it doesn't stretch, then it moves at the same time as you pull the end you've got in your hand.

It's this if it doesn't stretch. What's the plane on a treadmill thing then?

Ma'am · 30 Jan 2009 at 20:30

Le_Petit_Lapin said:
What's the point in a 10 mile long rod if you can't wave it about?

I was thinking more along the lines of - what's the point of a 10 mile long rod

Grrrrr · 30 Jan 2009 at 20:30

Dr.EM said:
At a complete guess, whatever the speed of sound is in the given metal?

I don't know the answer but i'd say this is the best guess so far.

It definitely wouldn't be instantaneous or at the speed of light

Dr.EM · 30 Jan 2009 at 20:32

A more avant garde view might be that it moves at the speed of electricity since it is electrons that give hardness to a material and they in effect could be thought of as transmitting the movement in this case. This would place it below light but above sound in speed terms.

moon man · 30 Jan 2009 at 20:33

and if you pulled it say 1 foot but the other end hasnt moved yet is the length of the rod now 10 miles and 1 foot?

Long rod question

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