Misconceptions and Fallacies

[vondo]

Anywhere.

You asked what would happen if you stood on a scale. If you are neutrally bouyant and "stood" on a scale that worked underwater, it would read zero. Since what a scale reads is the basic definition of weight, then you are weightless.

The scale in your bathroom doesn't measure your mass, either. It measures your weight. The two differ by the volume of your body times the density of air. Of course, you can basically ignore that.

Let me ask you this: Is an astronaut in the space station "weightless"? Why?

 

[plot]

You still have a force acting upon your body to keep it neutrally bouyant... your BC. Your BC is pulling you up while gravity is pulling you down... they just happen to be in a neutral state where you don't move anywhere. So, you're not weightless at all...

 

[Blackwood]

You asked what would happen if you stood on a scale. If you are neutrally bouyant and "stood" on a scale that worked underwater, it would read zero. Since what a scale reads is the basic definition of weight, then you are weightless.

If I stand on the scale in my bathroom, what is the net force acting on my body? Hint, I'm not accelerating.

That's right. The net force is zero, but I'm certainly not weightless. I weigh about 730N.

The scale in your bathroom doesn't measure your mass, either. It measures your weight. The two differ by the volume of your body times the density of air. Of course, you can basically ignore that.

nnnno. Mass and weight do not differ by buoyancy. One (weight) is proportional to the other (mass) by gravity. I don't weight less in a vacuum than I weigh in the atmosphere.

W=mg, not sum(F).

Let me ask you this: Is an astronaut in the space station "weightless"? Why?

Technically? Absolutely not. Why? Because they have mass, and are not an infinite distance from any other mass.

Per Random House Unabridged Dictionary, © Random House, Inc. 2006? Yes.

 

[rakkis]

Force = mass x acceleration

- Weight is a force measured in Newtons (N).
- g is a measure of acceleration due to gravity at sea level (approx 9.8 m/s^2).

Hence, as you mentioned:

weight = mass x g.

In a very real sense, you ARE accelerating. Your body is spinning around as it follows the earth's spin. You are just not moving in reference to the scale underneath you.

 

[SparticleBrane]

As much as people might hate Wikipedia, there's a great article on weight.
http://en.wikipedia.org/wiki/Weight

The weight force that we actually sense is not the downward force of gravity, but the normal (upward) force exerted by the surface we stand on, which opposes gravity and prevents us falling to the center of the Earth. This normal force, called the apparent weight, is the one that is measured by a spring scale.

For a body supported in a stationary position, the normal force exactly balances the earth's gravitational force, and so apparent weight has the same magnitude as actual weight. (Technically, things are slightly more complicated. For example, due to the earth's rotation objects are subject to a small centrifugal force, varying with latitude, which partially offsets gravity. The normal force therefore balances a force slightly less than the true force of gravity. These and other factors are explained further under Apparent weight.)

If there is no contact with any surface to provide such an opposing force then there is no sensation of weight (no apparent weight). This happens in free-fall, as experienced by sky-divers and astronauts in orbit, who feel "weightless" even though their bodies are still subject to the force of gravity. The experience of having no apparent weight is also known as microgravity.

So apparently it's acceptable to say that they have weight, but that their apparent weight is equal to their weight, thus they are neutral, aka microgravity.

Personally I would say that they don't weigh anything, assuming they are perfectly neutral: the force of gravity and the normal force from the air in their BC are in equilibrium so the net vertical force is 0.
Can you have mass and not have weight? No, because you can't be infinitely far from something else in the universe. However you can definitely be far enough away that it doesn't really matter.

It's kinda like that mathematician joke where a mathematician and a physicist are sitting around. Someone says that every minute they'll move the two guys 1/2 the distance to a nice steak dinner. The mathematician got very agitated...but the physicist remained calm.
Mathematician: "Oh no, we'll never get there. Why aren't you complaining, physicist?"
Physicist: "Because we'll get close enough that it doesn't matter!"

 

[plot]

Who hates wikipedia?

 

[SparticleBrane]

Plenty of people on this board. Personally I'd never use it for a master's or PhD paper but for most stuff it's a great reference.
As to this argument over weightlessness, it's ridiculous and stupid and I can't believe we're being this nit-picky.

 

[Blackwood]

In a very real sense, you ARE accelerating. Your body is spinning around as it follows the earth's spin. You are just not moving in reference to the scale underneath you.

Obviously, but centripetal acceleration doesn't impart weight.

Personally I would say that they don't weigh anything, assuming they are perfectly neutral

As I sit here typing this, my chair is pushing up on my butt with a force equal to the force with which gravity is pushing my butt into the chair (minus whatever buoyancy my body displacing air imparts and some other negligible forces). Hence, I'm in equilibrium and am every bit as neutral as I would be were I diving. Do I weigh anything?

Plenty of people on this board. Personally I'd never use it for a master's or PhD paper but for most stuff it's a great reference.
As to this argument over weightlessness, it's ridiculous and stupid and I can't believe we're being this nit-picky.

Yah, but it's fun, and webspace is cheap

Incidentally, I like wiki. I read it a lot, and there is an abundance of good information there. The reader must be weary, but that's true with any source, anonymous or not.

 

[grassyknoll]

Myth-"diving is an affordable activity"

Okay, pretty lowbrow, but after 21 pages of formulas and armchair theory I couldn't take it anymore.

 

[jeffrey-c]

Got to say this has been kind of an entertaining thread. For a recreational activity/sport that has a strong element of science (understanding of gas laws, bubble theory in the bloodstreams, etc.) there seems to be a littany of scientific misperceptions in physics (like some of the weight versus mass stuff), thermodynamics (water in a wetsuit keeps one warm, my favorite), etc. It's fun that there are so many intellectual discussions in SB on our sport, providing lot's of education for all - you sure don't see that on a tennis forum!