A Conversation Starter for Stan!

Topics not relating to derbies, regattas or cubmobile races.
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whodathunkit
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A Conversation Starter for Stan!

Post by whodathunkit » Tue Nov 30, 2010 8:07 pm

Stan, I'm no physics master like you are! That stuff is way over my head!

But can I pick your brain for a minute Please.

Stan, If I had a Pinewood Derby Car... And let's say it weighed in at
5.1 to 5.2 oz.

And I knew I couldn't race it here in the states.

So let's say I took the car to another country to race!
Because I knew weight depends on mass and gravity.
And that the earth isn't a uniform sphere, and gravity veries slightly over the plant.

And let's say I checked the car in at .. let's say Cape Town & Rome
Would my car weigh less in Cape Town
And be a little heavier in Rome but not by much.

Like i say this is just a Conversation Starter for you! Stan.
And I pray it does't go south!


What type of automobile can be spelled the same forwards & backwards?

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Stan Pope
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Re: A Conversation Starter for Stan!

Post by Stan Pope » Tue Nov 30, 2010 11:12 pm

I think that you are correct that the force of gravity at earth's surface is not constant. However, I think that the variance is much too small for you to find a "comfortable" place to race your car legally under 5 ounces. If you can find a track set up at the bottom of a very deep (and dry) well, the increased air pressure may create enough bouyancy that the car would pass. However, you would have to check on just how the scale was calibrated ... if it were calibrated with a 5 ounce block of wood, you would be in trouble. You would probably need to wear asbestos BVD's, too.

I think that there is a problem with the "well" solution ... I seem to recall solving some equations when I was in school about the attraction inside a sphere. You would expect to find g disappear when you reached the center (if you could reach the center.) And I think that it declined smoothly (as most natural measures seem to do) as you transit from surface to center.

Another issue that you must contend with is whether the rule limits mass or weight. Mass is independent of gravity (so long as you avoid relativistic conditions), so if the rule makers interpreted the limit as "reading 5 ounces on a scale calibrated by a 5 ounce standard purchased from BSA and measured at standard temperature and pressure (STP)" then you won't find a place to race.


Stan
"If it's not for the boys, it's for the birds!"

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FatSebastian
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Re: A Conversation Starter for Stan!

Post by FatSebastian » Wed Dec 01, 2010 7:48 pm

whodathunkit wrote:Because I knew weight depends on mass and gravity. And that the earth isn't a uniform sphere, and gravity veries slightly over the planet.
Weight is proportional to acceleration due to gravity. Surface gravity g has two primary constituents: gravitational acceleration due to the (slightly irregular distribution of) mass of the Earth, and centripetal acceleration due to the rotation of the Earth. Despite having excess mass at the equator, Earth's surface gravity is about 0.5% more at the poles than at the equator largely due to the fact that the equator is rotating relative to the poles. There is also a subtle reduction of acceleration with increasing altitude above mean sea level.
whodathunkit wrote:And let's say I checked the car in at .. let's say Cape Town & Rome
This page includes comparative gravity in various cities around the world, including Cape Town and Rome. Based on values from that table, a car that weighed 5.000 ounces in Washington DC would weigh 4.997 ounces in Cape Town and 5.001 ounces in Rome, due to changes in local gravity alone.

However, a properly calibrated scale will be insensitive to differing local gravity. If the same "5 ounce standard purchased from BSA" is always used in a balance scale, or the same standard was used to calibrate identical single-pan scales at Cape Town, Rome, and Washington DC, then the scales should all register the same readings for the same mass (except for the buoyancy effects as Stan notes). Ordinarily then, a car that is overweight in one locale will also be overweight in another locale having different g if it is weighed on a scale that has been calibrated locally.

Now, a car that exerts a downward force of 5.02 ounces in Anchorage should read 5.00 ounces on the equator if that car was checked-in using a single-pan scale calibrated in Alaska but not recalibrated after relocating to the equator. But even if one was allowed to take a scale from the arctics to the tropics and use it for check-in without re-calibrating, odds are it would still not precise enough to register the effect. Most scales in use seem to be postal-type scales that are only precise to 0.1 ounces (although inexpensive pocket scales precise to 0.1 g / 0.05 ounces are becoming more common). The most precise scale I have ever seen used for a BSA race was precise to 0.01 g / 0.005 ounces (it was my own), but most race organizers will not risk a sensitive high-dollar scale at a derby inspection station.
Stan Pope wrote:However, you would have to check on just how the scale was calibrated ...
Yes, it seems that air buoyancy can always affect scale accuracy, but the effect is usually pretty small! A high-performance car will tend to be made from a combination of very dense metal and some wood, and thereby its overall density should be comparable to that of the typical calibration standard (such as plated stainless steel), potentially minimizing that effect if a buoyancy correction is not accounted?



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whodathunkit
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Re: A Conversation Starter for Stan!

Post by whodathunkit » Thu Dec 02, 2010 8:49 pm

Wow! I still don't understand the mathmatical side of physics
Someday maybe it will soak in.
But like Stan said thats like tapping a dry well!

So can i please say Drats with a snapping sound of my fingers
looks like my perfect win record is still a big 0.
And just so you know guys i'd like to keep it that away!
(I'd much rather see the kid's winning this race!)

Call it dumb county boy craftsman ship.. But it's one I'll always
chalk up as a win when it comes to the kid's.

Sorry for the misunderstanding guys!
And Thank You for your Help. :whew:

And just so know guys... [...] a box is just a box!
And my well runs over with ice tea,milk,and soda pop!
And a Big Heart of love for this event!

This race isn't about us it's about the kid's!
the kid's make it fun but it's us adults that make it competitive!


But you know guys Does a grate leader always have to do good?
Time magazine awards the annual title of 'Person of the Year'
not to the person who does the most good,
but to the person who makes the biggest difference to the world,
good or bad. (And why is this?)

Seem to me: That the Lord should hold this title
for he is the true leader!


What type of automobile can be spelled the same forwards & backwards?

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