Physics idea

[Keroscene]

Re: Physics idea

But what if Mars doesn't appreciate being poked incessantly?

No, really, the "push" you give your rod (no pun intended) will travel to Mars at the speed of sound (not the speed of sound in air, the speed of sound in the material the rod is made out of.)

That is because such a push constitutes a compression wave in the rod.

Harte

When does it become a compression wave? If I hold a rod in my hand and thrust it out at something it's not creating a compression wave is it? I understand that the larger the something gets the less rigid it is, but the strength also depends on the material. We don't know of anything perfectly rigid that would let us do this, but would it be theoretically possible based on our understanding of matter to construct some type of material that is? On a smaller scale it would seem things are perfectly rigid so it makes me think there is a way to apply it to larger objects.


Way too many possible puns in this thread.

 

[StarLord]

Re: Physics idea

Kero, how would you propose the little guy keeps up with the big guy?

Also, Something "perfectly rigid" would stay in contact with Mars for how long?

Just saying.

 

[Grayson]

Re: Physics idea

But what if Mars doesn't appreciate being poked incessantly?Harte

The real question is; What if Mars does? HA! Never though of that did you Mr smartypants!

 

[TimeWizardCosmo]

Re: Physics idea

Just stay away from Uranus. That's a no poke zone.

 

[Keroscene]

Re: Physics idea

Kero, how would you propose the little guy keeps up with the big guy?

Also, Something "perfectly rigid" would stay in contact with Mars for how long?

Just saying.

I've been deliberately ignoring things like 'stay in contact with Mars for how long' and "the amount of energy required to move it" because I'm more interested in why matter can't be absolutely solid. Take the actual poking of any planets completely out of the equation and just think of the size of something. Remember that scene from Spaceballs with the five minute intro of just the underside of the ship? Was a play on Star Wars... Anyways there must be an inherent problem where if any object becomes to largg it will take a while for the other end to catch up. A ship one light year across ( or smaller with a nice round number only for example) with engines placed evenly the length of it... If they all fired up at the same time all parts of the whole move at the same time. Either side with engines firing up not in order would take time for the other side to catch up...at what size would the ship be where it would experience the phenomenon of one side moving before the other does? I think it's a pretty interesting question and one that might be thought of in the future if we ever make star destroyers. Do ocean going ships experience this phenomenon but at a smaller scale? Does the stern move faster than the bow? At what point does it become a problem?

 

[Harte]

Re: Physics idea

A ship one light year across ( or smaller with a nice round number only for example) with engines placed evenly the length of it... If they all fired up at the same time all parts of the whole move at the same time.

The problem with this is that, at that scale, there is no definition of "at the same time."Simultaneity , if that's how you spell it, is in the eye of the observer at all times. We just don't notice this fact when the scale is small.Regarding the flexing of large objects, that's an extremely complicated subject. I know it sounds simple, and the problems are easy to state and easy to grasp, but the actual calculations of such things are very, very complex.

 

[StarLord]

Re: Physics idea

The problem with this is that, at that scale, there is no definition of "at the same time."Simultaneity , if that's how you spell it, is in the eye of the observer at all times. We just don't notice this fact when the scale is small.Regarding the flexing of large objects, that's an extremely complicated subject. I know it sounds simple, and the problems are easy to state and easy to grasp, but the actual calculations of such things are very, very complex.

Ha! lets see the math.

Back to Kero,

"at what size would the ship be where it would experience the phenomenon of one side moving before the other does?
Wouldn't you need a two or three part ship that is connected by cables to each section and there's some play in the cables?
If a ship is a whole and not made of seperate parts, how can you have the left side or front, move faster or slower than the right side or back of the vessel?


I think it's a pretty interesting question and one that might be thought of in the future if we ever make star destroyers.
By then, they'll probably forgetaboudit "pushing" and instead PULL a craft by creating a contained singularity that you can adjust the mass by adjusting the power fed into it.
Kind of like FTL, Horse and carrot on a stick all rolled into one.

Do ocean going ships experience this phenomenon but at a smaller scale? Does the stern move faster than the bow? At what point does it become a problem?

If I'm not mistaken, there's some sort of physics involved here as it depends on the direction of motion. For instance, the new generation of steering of large Cruise Lines ship for instance and Ferry's use either Water Jet (like the jet ski) or Screw (nautical propeller) propulsion at the front, mid section and rear. So it's possible that the bow looks like it's moving before the Stern does, but only as much as the very center point of a watch hand where it connects the the time piece in relation to the tip as it circles the circumference of the watch face.
Sort of an tiny optical illusion?

 

[Harte]

Re: Physics idea

Okay, Mr. Smarty Pants (as Grayson so eloquently put it.) Here's something about flexing one short rod. It's an abstract from an engineering paper and includes no math.

The class of nonlinear tasks for flexible rods, even in the case of flat deformation, is rather complicated. Closed form solutions are possible only for some special cases. An algorithm of numerical integration of the nonlinear equations of equilibrium of a flat rod by a Runge-Kutta method is considered. The boundary conditions on the extremity of a rod are satisfied approximately. The projection equations are obtained from the common vectorial equations of equilibrium. As solving functions the projections of a transition vector and internal forces vector on global axes, angle of a normal's turn to the rod axis, and internal flexing, are chosen

Source
Don't have time right now to find equations for you. I dopn't remember much about this subject from my college days, but I do remember it was ugly. No matter. even if I could remember all the equations I yused to know, this board doesn't have the fonts and my computer doesn't have an equation writer that could handle the symbols IIRC.

When I can cut and paste, or link you to some examples, I will - so long as you're really interested and so long as you remind me to do so.

BTW, excellent Uranus joke Brent - we get so few of those!

Harte

 

[Harte]

Re: Physics idea

When does it become a compression wave? If I hold a rod in my hand and thrust it out at something it's not creating a compression wave is it?

We're pretending that you have the strength to move a rod long enough to reach Mars.

Are we gonna pretend it's weightless/massless?

When you push on this end, you're pushing against trillions of tons of mass. That's a lot of inertia - waaay more than enough for your movement to propagate as a wave along the rod as the inertia is overcome from here to Mars.

Now, your own (personal) experiences with rods you may have had in hand aside, I believe that in polite society, pushing rods around will cause quite a wave - albeit likely not one of compression.

And, also, ....

Uranus.

Harte