Physics / Math Challenge:

[fairsheet]

Yesterday up here in the PNW, the Tacoma Narrows Bridges were shut down, because of the danger of falling ice. These bridges are a major thouroughfare, so this is a big deal. Anyway...one of the backed-up cars was carrying a woman on her way to the hospital to have her broken wrist set. The police allowed this car to cross.

News reports say the car crossed at a "safe pace" of 30MPH (speed limit is 60). What would YOU have done? I would've gone 60+ to minimize my exposure too falling ice, even as if I'd encountered any, I'd have done so at twice the speed.

What do the math/physics say about this one?

 

[Spamature]

Well it depends. If you go faster. You might minimize your change of getting hit. But you maximize the damage of getting by a falling ice if you hit it.

Anyway this is kind like the question of you do run or walk to shelter in a rain storm.

Turns out walking gets you less wet.

http://www.wisegeek.com/do-you-get-wetter-if-you-run-or-walk-in-the-rain.htm

 

[fairsheet]

Well it depends. If you go faster. You might minimize your change of getting hit. But you maximize the damage of getting by a falling ice if you hit it.

Anyway this is kind like the question of you do run or walk to shelter in a rain storm.

Turns out walking gets you less wet.

http://www.wisegeek.com/do-you-get-wetter-if-you-run-or-walk-in-the-rain.htm

What Spam?....you can't fill me up a chalkboard with all sorts of functions, and equations, and Greek symbols that're Greek to me? -

 

[Lobato1]

30MPH to avoid aquaplaning or hydroplaning by tires on an icy bridge

Best Regards
Lobato1

Yesterday up here in the PNW, the Tacoma Narrows Bridges were shut down, because of the danger of falling ice. These bridges are a major thouroughfare, so this is a big deal. Anyway...one of the backed-up cars was carrying a woman on her way to the hospital to have her broken wrist set. The police allowed this car to cross.

News reports say the car crossed at a "safe pace" of 30MPH (speed limit is 60). What would YOU have done? I would've gone 60+ to minimize my exposure too falling ice, even as if I'd encountered any, I'd have done so at twice the speed.

What do the math/physics say about this one?

 

[fairsheet]

Best Regards
Lobato1

I should've clarified that the bridge surface wasn't icy. If it was that cold, the ice wouldn't have been falling off its uppers.

 

[Spamature]

What Spam?....you can't fill me up a chalkboard with all sorts of functions, and equations, and Greek symbols that're Greek to me? -

Here you go as close as I can find



http://askville.amazon.com/wetter-running-rain-walking/AnswerViewer.do?requestId=1760329

 

[Lobato1]

With dry road I'd speed but if LARGE ice chunks R falling I'd slow down

because the car's horizontal speed adds vectorially to the vertical downward speed of the falling ice.


Best Regards
Lobato1

I should've clarified that the bridge surface wasn't icy. If it was that cold, the ice wouldn't have been falling off its uppers.

 

[Lobato1]

BTW: Formula is square root of the sum of the squares

Best Regards
Lobato1

 

[PACE]

Dynamics transfer, and there is a thermal coefficient mismatch between steel and ice, so expansion occurs at different rates, the lower the dynamics transfer thru the steel structure, then the less that is transferred to the ice, so given thermal expansion (which requires offsetting) and lower resonant frequency, the safer one would be to travel at a lower speed. Material which has a CTE mismatch moves away from each other, as opposed to matching CTE, where the materials move together. Resonant frequency is a function of the stored energy in materials, the natural oscillation of the material itself.

Less vibration = lower "ringing effect" in the steel structure, and also the intrisic birefrigence of ice crystal formation, raises the random fracture or chip propogation. Crystal formation is random is many materials, especially ice, calcium floride and lithum floride, calcium floride and lithum both are hydoscoptic. Materials such as fused silica have less birefringence, therefore whether you are using it as a optic or a stablizing material.

Youngs modulus plays into this also, the tensile strength, the ratio of uniaxial stress over the uniaxial strain of a material. Bridges typically are designed to manage this tensile phenom thru the use of flexures such as trusses. so the "bounce" you experience when stationary on a bridge is less translated to you when the bridge structure has a the proper ratio of tensile flexureing. I-beams are designed to manage this well, the natural "bend" of a steel billet is offset by the hog out of the two loading bearing sides, creating a flange, which can offset the effects of torsion or the twisting of material.

Did I pass?

Regards
Pace

 

[RedCloud]

Dynamics transfer, and there is a thermal coefficient mismatch between steel and ice, so expansion occurs at different rates, the lower the dynamics transfer thru the steel structure, then the less that is transferred to the ice, so given thermal expansion (which requires offsetting) and lower resonant frequency, the safer one would be to travel at a lower speed. Material which has a CTE mismatch moves away from each other, as opposed to matching CTE, where the materials move together. Resonant frequency is a function of the stored energy in materials, the natural oscillation of the material itself.

Less vibration = lower "ringing effect" in the steel structure, and also the intrisic birefrigence of ice crystal formation, raises the random fracture or chip propogation. Crystal formation is random is many materials, especially ice, calcium floride and lithum floride, calcium floride and lithum both are hydoscoptic. Materials such as fused silica have less birefringence, therefore whether you are using it as a optic or a stablizing material.

Youngs modulus plays into this also, the tensile strength, the ratio of uniaxial stress over the uniaxial strain of a material. Bridges typically are designed to manage this tensile phenom thru the use of flexures such as trusses. so the "bounce" you experience when stationary on a bridge is less translated to you when the bridge structure has a the proper ratio of tensile flexureing. I-beams are designed to manage this well, the natural "bend" of a steel billet is offset by the hog out of the two loading bearing sides, creating a flange, which can offset the effects of torsion or the twisting of material.

Did I pass?

Regards
Pace

Uh, Professor, would you go that again , please!

 

[gabriel]

definitely not

 

[gabriel]

at 30 you have a far better chance of avoiding any ice you might see falling

 

[PACE]

Okay I put a container of ice (plastic container) in a tub of hot water, after a few minutes, I have an ice mold slightly smaller, but with clean sides, completely removed from the plastic, why? because heat transfer occurs faster in the plastic than in the ice, CTE mismatch, the materials expand at different rates, so the plastic expands and moves away from the ice.

Now the heat, which is energy transfers to the plastic at the molecular level, exciting the molecules thus they move faster and faster, vibration is energy, transferred thru steel, given the dynamics of the material and it's characteristics, or plastic deformation (impurities in the material structure reacting at different rates to energy transfer) suddenly I have a energized structure with a CTE mismatch that can cause ice and steel to move at different rates, so the ice falls given the vibration or energy transfer and suddenly I have a large ice structure thru the roof of my car.

or I can proceed at a lower speed, lowering the transfer, and after I've made safe passage over this dynamic structure, the effect of the transfer, delayed occurs after I am off the bridge.

Regards
Pace

 

[PACE]

I have to respectfully disagree, I see your point below, but the physics are sound here.

Regards
Pace

 

[gabriel]

they may be sound but have nothing to do with the problem. the ice will fall no matter what your speed

 

[PACE]

Ice falls because of G-Force but the physics behind the separation of the ice from the steel structure plus the energy transfer is what I refer to here.

If there were no energy transfer and the CTE was matched, the only effect of G-force would be "sag" but not separation necessarily.

Regards
Pace

 

[Corimini]

KE

In the end Kinetic Energy" KE = .5mv^2 where m is mass and v is velocity so if you double the speed the KE is increased fourfold - so yes - minimize chance (perhaps) by going fast but maximize damage. Anything else is pure guesswork since we do not know the rate at which the ice is falling. If there is one chunk per minute or 10 chunks per minute or 1 chunk per hour. Just change your friction tensor and you'll be fine.

 

[gabriel]

the ice is falling whether there is a car on the bridge or not

 

[Lobato1]

IMO: The problem is far simpler, for starters Ice is already falling from bridge

& assuming it's a large structure in relation to the car, making it independent of car vibrations.

The question is reduced to drive faster or slower through the bridge in the middle of falling ice & period.

​

Best Regards
Lobato1

 

[Lobato1]

You beat me to it.

the ice is falling whether there is a car on the bridge or not

Best Regards
Lobato1