Problem with the iron staff

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">[rousejohnny] Does elasticity of a material have an effect on the speed of sound or any wave through that material.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">Yes.
<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">[rousejohnny] Does thickness of the material have an effect? i.e. thin strip of aluminum verses a thicker one?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">Sometimes.

In general, the stiffer a material is the faster a pressure wave (such as a twist or a bend or a push or a tap) will travel thtough it.

But material thickness (of one dimension relative to the others) can lead to an object that is stiffer in one dimension than in another dimension. A steel bar 1 cm by 1 cm by 5 cm is just about the same stiffness in all three dimensions. But change that last measurement to 5 million cm and the bar can be tied into knots without creating any permanent kinks. As long as you don't try to pull the knot too tight, of course.

The speed of some waves can be influenced by this differential stiffenss of the object. An example would be my favorite toy, the slinky. Many are made of steel, and the speed of sound in steel is about 5 (?) times the speed of sound in air. But because the slinky is shaped like a spring it is possible to set up a wave motion in it that travels at a slower speed, approximately 2 to 3 meters per second. The exact value depends on how much tension there is in the slinky.
<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">[rousejohnny] Another thing is if you have a garden hose that is tangled and you flip a wave through it, it will move much slower than if you pulled one.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Hmmm. Lay the hose out straight. Pull on your end and have a friend signal when the pull gets to his end. Record the time required.

Now, push on your end and record the time required for the push to reach your friend.

The difference is caused by the amount of tension in the hose. Very high for the pull case, zero for the push case.

(Have you ever actually tried to throw a wave through a tangled hose? Try it.)
<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">[rousejohnny] What is the speed of a force with a flat amplitude vs. one with a high amplitude.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
What is flat amplitude? Do you mean low amplitude? But regardless of what you mean, in most media waves of all amplitudes travel at the same speed.
<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">[rousejohnny] This is why the speed of sound answer does not sit well. Is the light wave the flatest amplitude or is there something that can move faster such as a straight pull or push on a rigid object?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
If the speed of sound in the object being pulled is faster than the speed of light, yes. Otherwise, no.

Got it. Thanks Larry!

Johnny

Glad I could help.

Keep in mind that there are some details we didn't cover. Some time spent studying the chapters on mechanics in a sophmore level physics text might be helpful. Suplemented with the occasional question here or on one of the other messge boards.

LB

The point here is that there is in reality no macroscopic body which is strictly rigid. This is because the atoms have inertia and it takes therefore some time to react to any changes of the electrostatic force fields between the atoms. A rod with zero mass *would* transmit any movement instantaneously along its length (the speed of sound would be infinite). A realistic rod with a finite mass still reacts instantaneously, albeit only by an infinitesimal amount.
(in this context, see my page www.physicsmyths.org.uk/retard.htm where I showed that any static force must indeed 'propagate' unretarded as otherwise one would obtain different forces in different inertial systems (in contradiction to the definition of a force)).

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www.physicsmyths.org.uk
www.plasmaphysics.org.uk

Does it matter whether I Pull or Push the iron staff, The reaction of the atoms got to be faster if pushed together than if pulled away from each other shouldn’t it?

That means the impulse trough the iron would be faster if pushed rather than pulled. When it is being pushed the pulse would be equivalent of the speed of sound trough iron, how fast would it go if I instead decided to pull the staff?

I’m having a hard time imagining the speed would be the same.

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by PheoniX_VII</i>
<br />Does it matter whether I Pull or Push the iron staff, The reaction of the atoms got to be faster if pushed together than if pulled away from each other shouldn’t it?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">The wave speed is determined by the medium density (and the speed of molecular vibrations), and not at all by the strength of the disturbance that initiates it. The latter determines only how large the amplitude of the wave will be. And the speed of the disturbance itself determines only the wave frequency.

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">That means the impulse through the iron would be faster if pushed rather than pulled. When it is being pushed the pulse would be equivalent of the speed of sound trough iron, how fast would it go if I instead decided to pull the staff?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">This only determines if the wave starts with a condensation or a rarefaction. If these terms are unfamiliar, read a bit about wave mechanics.

The key point to remember is that no molecules actually move ballistically through the medium. They just oscillate in place. -|Tom|-