Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 13/10/2016 12:23:01

Title: If a crank is a light year away from the thing it turns do they move at the same time?
Post by: thedoc on 13/10/2016 12:23:01

Chris Carson asked the Naked Scientists:
   Hi, I love your show!

Consider a steel shaft with a hand crank on one end (for example, the composition and the means for rotating it is irrelevant), you turn that shaft and the other end (appears) to turn at the same time. Now consider a shaft of, oh lets say 1 light year, in length. If you crank the shaft at one end, does the far end (one light year away) turn instantly at the same time, or does turning the shaft propagate at a finite rate? If so, what is that rate? The speed of light?

Cheers Chris (from Canberra Australia)
What do you think?

Title: Re: If a crank is a light year away from the thing it turns do they move at the same time?
Post by: PmbPhy on 13/10/2016 13:20:46

Quote from: thedoc on 13/10/2016 12:23:01

Chris Carson asked the Naked Scientists:
   Hi, I love your show!

Consider a steel shaft with a hand crank on one end (for example, the composition and the means for rotating it is irrelevant), you turn that shaft and the other end (appears) to turn at the same time. Now consider a shaft of, oh lets say 1 light year, in length. If you crank the shaft at one end, does the far end (one light year away) turn instantly at the same time, or does turning the shaft propagate at a finite rate? If so, what is that rate? The speed of light?

Cheers Chris (from Canberra Australia)
What do you think?

No. That would assume the impossibility that the crank is a rigid body and such a thing is impossible. The speed of at which changes in the material propagates is dependent on the material and is less than the speed of light for all material. Einstein was the first to show that a perfect rigid body cannot exist.

Title: Re: If a crank is a light year away from the thing it turns do they move at the same time?
Post by: evan_au on 13/10/2016 21:38:06

The torque will propagate through the steel at roughly the speed of sound in steel, which is about 6 km/s, or 1/50,000 the speed of light.

1 light year is 9x10¹² km, so your torque will take about 1.5x10⁹ seconds to reach the earth, or about 50,000 years.

Quote from: Archimedes

Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.

It might just take Archimedes a long time....

Read more at: http://www.brainyquote.com/quotes/quotes/a/archimedes101761.html

Title: Re: If a crank is a light year away from the thing it turns do they move at the same time?
Post by: syhprum on 13/10/2016 22:27:26

On the old mechanical railway signalling system where semaphore signals were moved by apparently ridged rods the relatively low transmission speed was quite noticeable

Title: Re: If a crank is a light year away from the thing it turns do they move at the same time?
Post by: evan_au on 15/10/2016 12:11:47

Quote from: Chris Carson

If you crank the shaft at one end, does the far end (one light year away) turn?

A steel rod, 1 light-year in length, is a near-immovable object.
To turn it at a reasonable speed, you need to accelerate it with a near-irresistible force; or a fairly-resistible force applied for an incredibly long time.

If you apply too much torque, it will cause a spiral fracture at the handle.
If you apply a lower torque, it is likely thrash around, and perhaps twist itself  into a knot.

A steel rod 1 light-year in length  is more akin to a strand of the most delicate gossamer silk than a rigid bar.

...from Canberra, Australia