Physics Question (1 Viewer)

[Bryan X]

The heart of the idea is that what we percieve here on earth is often skewed.

Yes, it's widely understood that environment affects measurements.

When taking measurements you need to decide on a standard. If you change that standard (the environment for example) you can't then compare your new results to the old ones.

For example, one could say that as I am typing this message that I am at rest. One could also say that I am moving at 1,000 miles per hour (Earth's rotation) or that I am moving at 70,000 miles per hour (Earth's speed around the Sun). ALL would be correct. But if I am going to compare my speed to someone elses I must CHOOSE ONE standard to measure against whether it be the Earth's surface, the center of the Earth, or the Sun.

 

[Mark Fitzsimmons]

Sorry, but that was a poorly written question.

I strongly disagree. If the question would have been written as proposed by Bryan, many of us would have reached the same conclusion. However, by writing it as I chose, we had a discussion with a broad pool of ideas. Isn't that significant and valuable?

 

[Keith Mickunas]

Buoyant force is a product of air, as I understand it, it is much the same thing as wind resistance.

so air was effectively removed from the equation. Thus you mislead some people who were on the right track.

This was a bad question for your teacher to offer up, and unfortunately your presentation of it throughout this thread resulted in it being nearly impossible for anyone to get the "correct" answer.

 

[Rob Gardiner]

Keith is right. Here is the initial problem:

A pound of feathers contains more mass than a pound of nails.

 

[RobertR]

WELL stated, Rob.

 

[Chris Farmer]

Rob nailed it. A pound of feathers will MASS more then a pound of nails due to atmospheric buyonancy, but they will weight the exact same amount as long as they are in a constant envoronment. Since weight is a measure of force, not mass, and force changes depending on atmosphere, comparisons of weight between environments are meaningless. You'll weigh less on top of Mt Everest then at the bottom of the Dead Sea, but your mass is the same (obviosuly not accounting for sweat, food consumed, etc.). On the flip side, a single nail on Jupiter will weigh far more then I do on earth, even though it contains far less mass. The original question was in fact bogus. A pound of nails weighs exactly the same as a pound of feathers, although their masses may be different if they are in different environments. And, if they both start in one environment and then both move to a second, their weights may change and become different, but then you no longer have a pound of one vs a poun of another.

 

[Brad Porter]

So in summary,

1. A pile of nails and a pile of feathers, each with a weight measuring equal to one pound when exposed to the identical atmospheric conditions, may have differing masses if those atmospheric conditions produce a buoyancy force within the pile of feathers. In other words, the mass of feathers required to generate a downward force of 1 pound is greater than the mass of nails to generate the equivalent force.
2. When a pile of each of these items is measured to exactly one pound in a vacuum (with all other things being equal), the pile of nails and the pile of feathers will have identical masses. The mass of this pile of feathers will be less than the mass of the pile of feathers that weighed one pound when exposed to an atmosphere.
3. So if you take the pile of feathers that weighed exactly one pound (when measured in atmosphere) and take the pile of nails that weighed exactly one pound (when measured in atmosphere) and weigh each of them in vacuum, the pile of feathers will weigh more than one pound (because the offsetting buoyancy force is gone). The pile of nails may also weigh more than one pound, but not likely as much more than one pound as the pile of feathers does.
4. The mass of the pile of nails that generates a down force of 1 pound when measured on the surface of the Moon is much greater (what is it? ~6 times?) than the mass of the pile of nails that generates an equivalent force on the surface of the Earth. The same applies to feathers, jelly beans, diamond rings, and poo.
4. In either case, 1 pound = 1 pound = 1 pound. It may take differing masses of particular item to generate this downforce, depending on other interacting forces, but the downforce is the same. This was the point that everyone has tried to make throughout the thread. I don't see that anything useful or productive is being generated by your professor's statement. The wording of it is clearly incorrect.

Is this thread done now?

Brad

 

[andrew markworthy]

Ahem. Back on page 2 I wrote:

He explained to me that this is because of the buoyancy of the feathers. Because, all the scale does is measure how much force is pushing down on it, and because feathers are more buoyant than nails, it will takes actually take a greater weight of feathers to create enough force to have the scale read one pound. Because a portion of the weight of each feather will be suspended by the air.

isn't this just a little bit hair-splitting? The basic principle - that feathers float more than nails - was explained a long time back.

Incidentally, although the argument is true under some practical conditions, it hardly denies physics, since the problem is in the method of measurement, not basic theory.

 

[Cees Alons]

Mark,

Andrew hits the nail (or is that a feather?) on the head. The answer to your question should be "false", if taken in an appropriate way.

My physics teacher claims that a pound of feathers weighs more than a pound of nails.

Only the establishing of the collections of a "pound" of feathers and nails is left vague here. It could either mean its mass, or its weight, of course in either case found only by a (absolutely correctly performed) physical procedure. It would be a rather childish problem if we would to have to assume it wasn't "a pound" at all, whatever meaning, e.g. because it was established in a sloppy way, or if the methods were different for the two collections.
In other words: one simply must be allowed to believe that it's a real "official" pound in whatever acceptable way, and comparable to each other.

The statement then goes on saying that of those amounts the collection of feathers "weighs more". Well, it doesn't.

If weighed properly (according to proper physics), it would of course weigh the same. But there's some room here for meaning a common, more or less lazy, not to sophisticated way of "weighing": but as already established in this thread now, and even explained by your teacher, that would lead to the pound of feathers (virtually) weighing less, not more.

Cees

 

[Julie K]

The big picture is this idea of perception and the inacuracy of many of the physics formulas we commonly use.

The big picture is that your teacher has done a piss poor job in trying to explain the difference between weight and mass.

 

[BrianW]

Andrew hits the nail (or is that a feather?) on the head.

...

Rob nailed it.

I think there is way more than a handful of people here who indicated early on that elements in a problem should be what they are represented to be, and that environmental factors that affect measurement should be accounted for when taking the measurements.

On the other hand, what a great way to enjoy a glass of wine late at night!

Julie, you need to keep an open mind. In a month or so, I'll be able to prove to you that your MER-A and MER-B Mars Rovers don't weigh as much as you think they do. Can you imagine what physical laws and formulas I'll have to bend in order to accomplish this feat? :rolleyes

(On a personal note, I can't wait until tomorrow's launch of the Beagle 2, followed later in the month by your very own pair of rovers. It's all very exciting, indeed!)

 

[Julie K]

In a month or so, I'll be able to prove to you that your MER-A and MER-B Mars Rovers don't weigh as much as you think they do.

Then it's a darn good thing that I don't care what they weigh. Their mass, however, is of great interest...

 

[Mark Fitzsimmons]

I am getting the feeling now that people are calling me ignorant and accusing me of not knowing the difference between weight and mass. As stated in my initial explanation that this is not a thesis defense as I do not wish to be drilled with questions of people accusing me of being wrong. If you would scroll back to my very first post opening this thread, I stated that I felt that they both weigh one pound. Many of you are angered over how the question is worded, but what is the problem? I have always believed in and ends based philosophy, if the ends are desirable, and the means to get there are moral, then it is a justified action. That is how I have always conducted my life and I think the same philosophy applies to this thread. I posted an open and thought provoking question, and it had turned into four pages of conversation that I am sure most of you enjoyed.


andrew markworthy:

Ahem, I was not trying to be misleading. I said that the 'experiment' should be done under the absence of wind so the feathers would remain on the scale and not be blown off. I did not say that air would no longer be a factor.


Julie K:

I would appreciate you not assuming I do not know the difference between mass and weight. That is rather condescending. I am not going to spew my credentials at you because it is rather irrelevant, but I am indeed not ignorant.


Rob Gardiner & Keith Mickunas:

I agree, the question would make more sense worded that way. However, I passed the question onto the community worded exactly how my teacher worded it. Don't kill the messenger.


A few closing words. You guys are saying all of this stuff like "the original question is bullshit" and I do not know what your intentions are here. Are you trying to tell me that I am wrong? Are you trying to say that I am stupid or uneducated? I want everyone to understand that I can not agree more that the question is bullshit. But as I stated in my first post that I believe both we weigh a pound, this is because weight is subjective, and as we all know changes depending on environment. I think that the problem is interesting because after examining the scenario it teaches us that the true weight of an object can often be different than what a scale (even a perfectly calibrated one) will read in our atmosphere. And that formulas like w = mg is only accurate in a vacuum, and rather we could more accurately name the formula weight = mass * force and calculate force as gravity with other forces like the buoyant force added and taken into account.

I do not know what peoples opinions of me are at this point. It seems that certain people may be angered at me for posting this thread, but remember these few critical things.

1. I was not trying to be misleading
2. I did not write the question, I just passed it onto the community exactly as it was given to me.
3. I agree that it is bogus. When the question was asked on my physics final, I wrote why it is an improper question.

 

[Julie K]

rather we could more accurately name the formula weight = mass * force and calculate force as gravity with other forces like the buoyant force added and taken into account.

More accurately,
Force = mass * acceleration.

Whenever you write an equation it is helpful to check the units. Remember, in Imperial units your mass is not measured in pounds.

(Gads, sometimes I feel like such a slug.)

 

[Keith Mickunas]

This is physics, not philosophy. At one point you said you were understanding his reasoning, thus you seemed to be agreeing with him. Perhaps it is merely your teacher that is ignorant of mass not being equal to weight, but at times you put forth his answer as the truth, even though substantial evidence had been provided against it.

The fact is, this question doesn't provoke much thought, it just gets people to look for a trick in the wording, or to show where your teacher was wrong, which was done early on.

 

[Cees Alons]

Mark,

No one makes any assumptions here about how clever or educated your teacher is, or you are for that sake. Nor do we try to attack you.
Most of us only try to point out that you were right, according to us, initially, when you said you thought your teacher to be wrong.

Indeed, he was wrong. If he forms a pound of feathers and a pound of nails in any honest way (honest to the stated problem), the feathers would weigh the same as the nails, or perhaps less (but never more), depending on how accurate or careless his definition of that "weighing" is.

Period.


Cees

 

[Mark Fitzsimmons]

I posted this thread because I wanted to know if my teacher was right. As Keith pointed out, at the beginning of the thread I thought it was bogus, then I said that his reasoning seemed to make sense. But ultimately, I was seeking a more educated community to find the truth. That is all.

I never claimed my teacher to be clever or educated, in fact, many times I have said quite the opposite about him. He is perhaps, the most ineffective educator I've been exposed to in my life. But this question did pique my interest, so I posted it.

I posted his question because I thought it would make a good thread. If people are mad at me, I'm sorry you feel that way.

 

[Cees Alons]

Mark, once again, we're not mad at you!
We think you have a teacher who doesn't think too well, now and then, and likes a spectacular, non-intuitive answer to a problem so much, that he even prefers it when the proper answer is the intuitive one.

Cees

 

[BrianW]

Like I said, it's all in good fun, Mark. And if it weren't lively, it wouldn't be nearly as much fun. Thanks for letting us play. Next time, I'll bring the football.

To Julie: Sorry, I know Beagle 2 isn't going up for two more days - it's just that I don't know what today is. For some reason, I thought today was June 1st. Here's hoping for no more delays to two perfect launches in June. (And here's hoping you find your Cyclopean cities with the angles all wrong.)

 

[Julie K]

It's ok Brian, I'm a bit fuzzy on Beagle as well. I'm in a mode of 'days to go' til the 8th.

Anyway, we're going to the wrong place if we want to find Cyclopean cities with wrong angles But it's ok, I still get immensely excited over red colored rocks.