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Just a simple problem

  1. Feb 25, 2004 #1
    Just a problem I thought about the other day. It’s just curiosity to know the answer, I’m not a physics expert (my first post also).

    Let’s imagine we could build a straight bar or something similar which is 1 light-day length (or any other huge length) made from any solid material (ie: fiber optics, cast iron, etc). For simplicity imagine the bar to be in empty space and let’s make the bar so one could grab one end and begin pulling it with some acceleration for some time (just imagine you have the means for doing it), so the bar will begin moving (in theory). The other end is still in empty space.

    Since nothing can travel faster than light, would the other end of the bar remain motionless for a day waiting for the energy that begins pulling it (going at light speed, the energy signal will arrive one day later), and consequently the bar would probably just break at some point (remember one end is moving because I’m pulling it)? Is this due to mass inertia? (Is mass inertia due to speed of light constraints over mass?)

    If the previous reasoning is wrong, would the bar move all at the same time? If so, how the other end of the bar received the energy so fast to begin moving?

    Thanks for your time.
  2. jcsd
  3. Feb 25, 2004 #2
    In my opinion there would be sort of a “pull-velocity wave” traveling through the bar, so the far end of a long bar won’t move until that pull energy is transferred through its molecules throughout its entire length.

    Long trains have to consider a similar problem, but mainly with trains this is due to the little gaps of space in between the couplers on each car. If a long train is somewhat “loose” all along its couplings, and if the engine starts out fast, it can move several feet before the last car stars moving. You can actually hear the loud “bump, bump, bump” of the coupling “wave” traveling down the train, when all the couplings in the front of each car get jerked forward by the rear coupling of the car in front of them.

    The bump of the sudden pulling of the first set of couplers, located between the engine and the first car, isn’t so hard, but the jerk between the couplers of the last two cars is very forceful, and if the train starts out too fast, it can actually break apart at some of the rear couplers.

    So, I would say that you would have to start your long bar off very very slowly so it won’t snap somewhere along its length, and that will allow time for the molecular-pull wave to travel through the bar. But, I think it might take months or even years for the wave to travel that far through the bar, from one end to another.

    A sound wave in steel travels at about 4512 meters per second, which is about 2.8036268 miles a second, and that is considered to be very fast. A pull-wave might travel faster, but still I think it might take some years for the far end of the bar to start moving.

    Let’s see, 2.8 mps is about 10080 mph or 241920 mpday.

    A light day would be 16070400000 miles, and so I figure it would take a sound wave about 181 years to travel that far through the steel. So when you start your pull, you might want to make it a pull of about 1 or two inches every 181 years, so you won’t snap your bar.
  4. Feb 25, 2004 #3


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    Staff: Mentor

    As with all objects, force is transmitted by pressure waves at the speed of sound through your iron bar. The end opposite from the one you are pusing on will start moving when the pressure wave gets to it.

    Ever hear of the "water hammer effect?" When you shut a valve quickly, the water next to the valve stops pretty much right away while water at a distance away is still moving toward the valve. The water molecules bang into each other, sending a pressure wave backwards through the pipe, stopping the flow and creating a loud banging noise in the pipes.

    Another analogy: a spring. Every object is a spring. That is, every object has a certain elasticity to it. If you put a spring against an object and push at a constant speed, the force is first transmitted through the spring, compressing it, then to the object, then the spring rebounds and eventually ends up the same shape it started.
  5. Feb 25, 2004 #4
    Why do I observe the "water hammer" effect mostly with the "hot" faucet? Mayhaps the hot water heater is more liable to cause resonances, or the speed of sound in hot water differs considerably from that of cold water.
  6. Feb 26, 2004 #5


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    Yes one light or probably more you will wait to get the signal on the other side.
    First of all,you don't have to think of something like metal (!) bar 1 light year long to see what happen.
    You may imagine let say the array of hydrogen* atoms put in a perfect line 1 light year long or something similar to see what would happen.
    When you absolutely preceise apply an impulse** of force to the first atom it will transmit that impulse to the second,then second to the third etc.
    So the basic question is how fast the impulse is being transmitted between first and the third atom in the array and how?
    Nothing can uniformly travel faster than light so the interaction needs at least the time which would light need to travel 1 atom lenght.
    How does that happen?This interaction is electromagnetic interaction between electron clouds of the adjanced atoms.Like the majority of body colision interaction in everydays life.
    * hydrogen choosen becouse of Einstein and this post made me laugh
    **1-dimensional momentuum exchange between two particles
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