Science Question - Argument with wife.

Of course.
see now that why we keep you around Francis.

All the air has been taken out of my argument.
Joe(my lips are pursed.)Nation

I think so. Try this...

1. A jar full of vacuum, in a vacuum, weighs as much as the jar.

2. A jar full of air, in the same air, weighs as much as the jar.

If it's the same jar, then the weight in both cases is the same.

My guess? They weigh the same, because it's the environment that creates weight.

Well, you're not at sea level, dp - when you're at home! Can you make any use of that?

Mind you, that whole difference goes out the door if you take the jars to the beach - with or without Francis' device!

Just follow The Rules - it saves the discussion going along tracks which you may find uncomfortable at some point.

Nope.

This is only correct if the glass of the jar has no mass or volume.

Yeah but she doesnt know that. . . SHHH!

The scientific answer is that the jar with air has a great mass.

Weight = mass * acceleration

This means the jar with air does weigh more. But we are not finished. The Net forces on the Jar also include the force applied by buoyancy. Buoyancy is a matter of volume in the jar and the ratio of the density of the fluid inside the jar versus that outside. If the fluid in the jar was less dense than the fluid outside, their would be a force acting opposite the direction of gravity. The normal force then the jar full of air would apply to a weighting device would be less than the vacuum jar. If however, you were to weight both jars in a vacuum sealed room, the force of buoyancy would be nullified and the matter (air) inside the jar would only contribute more weight.

Depending on the environment, the net force chances, the weight does not. Your wife won. Weight is only on of many forces that can act on the jar. Adding buoyancy does not change the weight, it only acts against weight.

A
R
T

Who does the vacuuming ? If the man does it, it has more matter left behind....

Are you saying there is no matter in it ?????

Same temperature , the air .

There is a win to be salvaged.

1) The wife is right.
2) Telling wife she is right is (and you are wrong) is...
a) right.
b) a faster resolution.
c) the best outcome regardless.

Naaa...he should stick up for his rights and take his flogging.....screaming and crying like any man would .

Failures Art,

With all due respect, there are a couple of problems in your post.

First of all, your use of Newton's second law is kind of appropriate (since weight is a measure of force). But the jar has weight even when it is not accelerating. Now you might point that the "acceleration of gravity" makes this equation correct. But this is a circular argument. You would be saying that the acceleration that happens to occur due to gravity on the surface of the Earth is determined by the exact force needed to cause the acceleration that happens to occur due to gravity on the surface of the Earth. There is a more relevant equation that determines this force that depends on the Mass of the Earth and our distance from its center.

Second, buoyancy is a matter of one thing. The buoyant force is equal to the weight of the fluid (i.e. the air) displaced by the jar (i.e. its volume). That is it. Since this is a rigid jar, meaning its volume doesn't change, the only thing that matters is the size of the jar.

What is inside the jar has no effect on the buoyant force.

As I see it,
the dispositive consideration
is the fallacy that, for some reason,
the force of gravity DISCRIMINATES against the matter of the air inside the jar
and will NOT pull down on it.

The force of gravity pulls down on ALL matter, be it solid, liquid or gas.





David

With respect. Read again.

1) Even at rest, Gravity applies a force.

F(gravity) =! 0;
ergo: mass*acceration(gravity) =! 0;
egro: neither term can have a value of zero.

It is not a circular argument. Newton's law is not "kind of appropriate," it is very much the law here. Mass is constant, weight is not. Weight, as a defined physically is only one component of the net forces acting on a body.

Net Force =! Weight;

2) Both the air jar and the vacuum jar displace the same amount of atmospheric air. Since the vacuum jar has zero additional mass, the ratio of air-mass it displaces is even higher, hence a greater buoyancy.

What is inside the jar absolutely matters. Buoyancy is a matter of volume and density, and so even if a fluid has a specific volume, the density will determine the force of buoyancy. The lower the density inside, and the greater the density outside will create a greater buoyant force.

E.g. - Fill two balloons to equal volume in a room filled with air. Fill one with air from the room, and fill the other with helium. The helium balloon will rise due to buoyancy. It is because the helium balloon's contents are less dense (mass/volume).

The net force acting on an object at rest (velocity==0) by gravity and buoyancy:


What is inside does matter.

In fact, nothing inside matters! If you could make a lightweight rigid structure and create a perfect vacuum inside, it would float!

At sea level:
1L of air= 1.25 grams
1L of helium = 0.18 grams
1L of vacuum = 0 grams

Same volume, different mass, ergo different density. The greater the difference in density, the greater the force of buoyancy. A vacuum would float better than helium.

A
R
T

He who fights and runs away, lives to fight another day.

*Runs.

Failures Art,

First of all, please give a definition of the word "acceleration"?

From that definition please answer these questions.

1) Is is possible for an object that is remaining at rest to have a non-zero acceleration?

2) Is it possible for an object that has multiple forces acting on it to have a no (or zero) acceleration?

Your answers to these questions will be very important to any further discussion on this topic.

This is the correct equation for net force. This is why they made the variable name Fnet. The net force is the total of all forces on an object which in this example (and you really should have understood the situation the equation was explaining before you posted it) is 0 which is simply means that it is not accelerating.

The "buoyant" force (which is what we are discussing) is the second term in that equation. You will note that this is determined by density of air outside the jar (the f stands for fluid) times the volume of air displaced times the acceleration of gravity (not again that nothing is accelerating at this rate, this is just the rate an object would acceleration in the even that we dropped it with no other forces).

What you will note about the buoyant force (i.e. the second term in the equation you posted) is that The only thing that matters it the density of the air outside the container, and the Volume of the container (since the acceleration of gravity at a point on Earth doesn't change)

So, as I said. The contents of the container, in a rigid container such as a glass jar where the volume doesn't change, doesn't have any effect on the buoyant force. Of course, as you put more air or helium in a balloon, it gets bigger (i.e. a greater volume and more air displaced). So this is an important way that a balloon is different than a glass jar in this problem.

Of course the first term of the equation is the container's weight (disregarding buoyant force). This of course is effected by the contents of the container as it adds mass. But that just tells us that a jar filled with air will have more mass (and more weight when it is on Earth) than a jar with a vacuum. And that is the whole point of this discussion.


For an expanded explanation of buoyancy, I will refer you to to the quite good wikipedia article which succinctly states



http://en.wikipedia.org/wiki/Buoyancy

Look! over there, two flies crawling up a wall!

Wanna argue about them?