How does balsa wood's weight change with altitude? URGENT!

Hi all,

I'm in a bit of a pickle here and need help figuring out a problem. In 10 days I will be traveling to a National competition to race CO2 powered dragsters. Some fellow students have warned me that because of the change of altitude(341 feet above sea level to 5280 feet above sea level) that the mass of my balsa dragster will change. This is important becuase there is a minimum weight the dragster must be(45g). I've been told that the difference will be 1.5g-2g below what my car weighs now.

The problem is that my dragster currently weighs 45.25 grams and I have been trying to calculate the exact diffenrence to avoid be disqualified or putting on too much weight. F=ma and the fastest car wins.

Using a change in air density and a change in gravitational energy I've come up with a difference of only .10g!

Does anyone have any further insite into how to calculate a accurate value?

I appreciate anone's help,
FlyBoy38L

If you need info here is some:

Volume of Balsa: 244 cubic cm
Mass of balsa used: approx 15.5g
Balsa Density: 4 Lbs/Ft3
Balsa's % of air content: 92%
initail altitude: 341 Ft above sea-level
final altitude: 5280 Ft above altitude

 

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Why don't you just weigh it when you get there?

 

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Probably it's pretty hard to calculate these things. I'd just take a bunch of 0.1 g weights with you and be ready to tack them on if you need to. Also, things like humidity will change, too. You're at sea level, and presumably you're going to Denver (I guess). Either way, the higher in altitude you go, the less humidity there is, so your balsa wood will dry out and be lighter. This is probably a bigger effect than the air density changing.

 

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If your dragster "weighs" 45 grams at the bottom of Death Vallley it will weigh 45 grams in Denver, 45 grams on the surface of the Moon, and 45 grams on the International Space Station. "Weight" is a term with multiple, conflicting definitions. Legally and colloquially, weight is a synonym for mass. Since you said the dragster has to weigh 45 grams or less, this is most likely what the rules committee means by "weight". If you are using a balance scale to determine the weight of your dragster you are probably in the clear. When physicists use the term weight they are talking about force (in two conflicting ways!) It is one of these two alternate meanings to which your friends were alluding. These meanings do not apply if the spec truly is 45 grams.

 

"Why don't you just weigh it when you get there?"
I'd just like to prepare for what kind of difference I'll be encountering and would like to try and calculate it. I know I will deffinitly be weighing the dragster before it is handed in for inspection.

"You're at sea level, and presumably you're going to Denver (I guess). Either way, the higher in altitude you go, the less humidity there is, so your balsa wood will dry out and be lighter. This is probably a bigger effect than the air density changing."
I actually am going to Denver, nice guess! As with the weights, I can't attatch any other things to the car other than wheels, axles, and bushings and I've already met the maximum of those numbers.
Any idea on how to calculate the loss of water mass becuse of the humidity change?

To DH,
I totaly forgot that weight and mass do mean completly different things in definition. I guess I'm trying to find the loss of air mass due to elevation and loss of water mass in the form of humidity due to elevation.

To All,

Where should I start in trying to find the lost mass of water due to humidity?

Regards,
FlyBoy38L

 

Your balsa will be lighter after the trip if exposed to circulating cabin air, because humidity is extremely low in the stratosphere. If you're trying to lighten it, you could (if you're carrying it on) air the dragster out while at altitude- the moving air would dry it faster- then package it to prevent moisture from re-entering the balsa. A water-absorbent pack in a small container would probably do as better over the whole trip.

If you keep and carry your dragster in a sealed plastic container (such as Tupperware) you can maintain or change the water content as you wish. You can find airtight plastic boxes of all shapes and sizes at a Wal-Mart or wholesale food store. If you don't have silica gel, you could bake some table salt in an oven (to remove moisture) and enclose that for de-humidifying. Conversely, you could add moisture to the wood by enclosing a damp sponge. If it's at the weight you want, just keep the car sealed from the environment as much as possible- like you would keep a sandwich- and its water content can't change for as long as it's sealed up.

 

This does not depend on altitude or even where you are. Humidity changes from day to day and can change a lot in a short time. Trying to calculate this is not something I know how to do, but you may want to compare the average humidity of where you're going with where you are now to get a rough idea of which way and how drastically the mass of the wood will change.

I doubt that the change in mass due to the change in height above sea level will be that great.

 

That's right- there's no mass change due to altitude. High air is just "freeze-dried": It is very dry, because it has been very cold. Warmed up by an airliner's engines, cabin air is very arid at altitude. Which could be handy for drying, although just a hair-dryer used anywhere, to waft warmed ambient air over the wood would do as well.

I've done a lot of model airplane building/flying/repairing and dealt with these changes- Areas of a balsa model that lack air circulation don't rapidly change mass with humidity, but structures or components that do have air circulation do change as they wick water vapor in and out. So if you're at average humidity and have the precise weight you need, package it air-tight. Whenever you take it out of the packaging (even in different humidity) it will stay at the same weight for some time. Just get your dragster to the weight you need, and treat it like a sandwich (except for eating it).

 

There is a small reduction in weight (NOT mass) due to altitude, since you will be 1 mile further from the center of the earth. It is approximately 1/2000 times the sea level weight or about .02 grams.

 

im assuming your a science student doing a science subject. If thats the case when they "weigh" you car, if its not the same weight that it was when YOU weighed it (taking into acount the acuracy or inacuracy of your scales) then tell them to reajust there scales because you cant just take a set of bath room scales to the space station and still expect to get the same result. ALL mass measurements should be ajusted to "weight" at sea level. There for tell them to stick it

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i'm almost positive you are wrong here DH.
it's the mass that remains the same, not the weight.
a 6 pound object on earth will weigh 1 pound on the moon.

 

leopold99

no your making the common mestake of mixing up the meanings.

MASS is measured in pounds, Kg's ect
WEIGHT is measured in Newtons.

Now if you took a set of bath room scales which havent been adjusted and stuck them on the moon they would measure you as being lighter yes but thats because scales dont directly measure MASS, they measure the force you mass aplies based at sea level based on the formular F=ma. The a in this case is the acceleration due to gravity (9.8 from memory). However your "weight" in the common sense of the word (ie your mass) remains at whatever it was no matter where you are, its just the acceleration due to gravity which changes. As they have stated the requirement in kg not in Newtons they are using the common not the scientific usage of the word and there for if it "weighs" differently they need to reajust there scales. take a 1 kg weight with you to prove this point

 

There is a change in weight when the term is used as defined in physics. There is zero change in weight when the word is used in either lay or legal parlance. Colloquially and legally, weight is a synonym for mass. It is not a force. That the contest specified a weight limit as 45 grams gives a big hint as to which meaning applies. The gram is a unit of mass.

This, however, makes zero sense. The gram is a unit of mass.

Pounds are a unit of mass, at least in the US. If you meant the unit of force, you should have said pounds-force, not pounds.

Weight is a synonym for mass, at least in the US. Here is what the National Institutes of Standards and Technologies says on this matter:

 

WIKI:

answers.com:

it appears to me that weight and mass are two different things.
it also appears that weight is the action of gravity on mass.

 

yes leopold99 ?

as i said if your measuring in kg's your measuring mass which is UNEFFECTED BY GRAVITY. Your own quotes surport that

Weight is however used in everyday speach incorrectly to refer to mass which it has been in this case. The way to see which they are using is to look at the units, if its Kgs its mass, if its Newtons its weight in the scientific sence

 

The reason why people find mass and weight so confusing is that when you want to measure the mass of something (in kg) you put it on a scales, which assumes the acceleration due to gravity is whatever it is. Obviously, if you took a set of scales that are calibrated for use on earth to the moon and tried to measure the mass of something that was really 10 kg you'd get a result that's around 1/6th of that. Does that mean that the mass changes? - no, it means that the measurement is wrong.

 

The pound is actually a measure of mass. The Imperial system has multiple measures of mass. A quick check: Google the word pound. The first thing you will see is

1 pound = 453.59237 grams

More about calculator.​

The Imperial system has multiple measures of mass. Wikipedia has a nice chart that depicts them.

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When used without any qualifier, the term pound technically refers to mass. Which pound? It depends. A pound of feathers weighs quite a bit more than a pound of gold. The "pound" in a pound of feathers is the avoirdupois pound: 453.59237 grams. The "pound" in a pound of gold is the troy pound: 373.241722 grams. Precious metals are weighed in troy weight. See the above chart.

What about force? The imperial unit of force is the pound-force.

The slug is one of two ways in which people who are stuck with the imperial system address the fact that F=ma is not true in the standard imperial units. Newton's law says force is proportional to, not equal to, the product of mass and acceleration. In short, F=kma. The metric system was designed from the ground up to have the proportionality constant be exactly one. The slug is not an imperial unit. It is a very late add-on (circa 1902) to the imperial system that lets us say F=ma once again. The other approach to enable saying F=ma in imperial units is to use a non-imperial unit of force, the poundal. AFAIK, nobody uses poundals.

The US aeronautics industry is inextricably wedded to the imperial system. Interestingly, they express vehicle weights in pounds (rather than slugs) but moments of inertial in slug-feet[sup]2[/sup]. Go figure. Even better: Go use the metric system. Unfortunately, demands to do so have fallen on deaf ears in the aeronautics industry.

 

Whether a scale on the Moon will register a 10 kg mass as weighing 10 kg or 10/6 kg depends on the type of scale used. Use a balance scale (the kind of scale used by chemists and in doctor's offices) and the 10 kg mass will register as 10 kg. Use a spring scale (the kind of scale found in people's bathrooms) and the 10 kg mass will register as 10/6 kg.

So WTF is weight? As I've already noted, legally and colloquially, weight is a synonym for mass. Physicists and engineers use weight in a different sense, and actually two different senses. Most elementary physics texts (up to freshman physics) teach us that weight is mass times the acceleration due to gravity.

Spring scales do not measure this second definition of weight. Some college-level physics texts and most general relativity texts use a third definition of weight: The weight of some object is that quantity measured by an ideal spring scale.

• Newtonian point of view: A spring scale measures the net real force acting on object less the net gravitational force acting on the object. In short, a spring scale measures the net non-gravitational force acting on an object.
• General relativity point of view: A spring scale measures the net real force acting on an object. The gravitational force is a fictitious force in GR.

 

Correct me if I am wrong

to Asguard,

Yes I am a science student. I actually just graduated High School and plan on majoring in mechanical and aerospace engineering. I compete in scinece related events and I take them very seriously as this is the last year I can compete(grades 6-12 only). I JUST LOVE SCIENCE! (Physics the most)

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To all,

I did some research and learned a bit about the atmosphere. Turns out the humidity that the weather station gives us is a ratio. I also found something I'm not sure of, doesn't absolute humidity change with elevation? Doing some more research I found that relative humidity is only a ratio of (water mass)/(air mass). This ratio is also equal to (relative humidity)/(barametric pressure). spelling? Given that the balsa contains 92% air shouldn't I be able to calculate the diferences in water mass of the air contained in the balsa?