four forces

[Fish]

Someone today tried to tell me that the four forces acting on an airplane are not lift, drag, thrust and weight, but in fact lift, drag, thrust and gravity.

Comments?

[niss]

I would think that weight is just another way of saying gravity and if you wanted to get technical weight is not a force, it is a byproduct. Gravity being the actual force.

[Fish]

Yeah, I said that Fg is force due to gravity which is the same thing as weight.

Where are the physics buffs?

[bob sacamano]

I would think that weight is just another way of saying gravity and if you wanted to get technical weight is not a force, it is a byproduct. Gravity being the actual force.

I realize you're a student, but a piece of advice, do your homework before you give out any lessons

As for the question,

Weight is a force, and that force is a vector quantity having both a magnitude and a direction associated with it.

[TopperHarley]

In physics terms, weight= mass * gravity. On earth, the value for gravity is 9.81 m/s2. It varies from planet to planet. Something has "weight" if the force of gravity is acting on it. On the moon, for instance, since the force of gravity (acting vertically downwards towards the center of the earth) is much lower compared to the earth, you actually "weigh" less on the moon than you do on earth.

[Fish]

In physics terms, weight= mass * gravity. On earth, the value for gravity is 9.81 m/s2. It varies from planet to planet. Something has "weight" if the force of gravity is acting on it. On the moon, for instance, since the force of gravity (acting vertically downwards towards the center of the earth) is much lower compared to the earth, you actually "weigh" less on the moon than you do on earth.

So the only thing that would change the weight is being on a different planet?
So was this guy right or not?

[Kilo-Kilo]

These guys changed their weight in relation to the aircraft and I'm sure the aircraft was near earth.

[jjj]

C-HRIS

You postulated that Weight = Mass x Gravity. I would appreciate if you could provide a reference for this.

This brings up a few questions:

If weight is in Kilograms and Mass is in Kilograms which I multiply by a constant with the units of meters per second squared - how can I possibly get a result of my weight on Earth in Kilograms.

Correct me if I'm wrong but perhaps you're thinking of the Force acting on a body (F) is equal to it's Mass (m) multiplied by acceleration (a) :
F=ma

Multiplying mass in Kilograms by a value in m/s squared would then give you a value in Newtons (N) - ergo what we know as G's.

9.8 m/s squared is the rate of acceleration due to gravity of any mass in a vacuum as it falls toward earth.

This brings up another fun point of discussion that touches on relativity and objects in motion - consider a closed system - me sitting here in front of my computer where I'm at rest and not falling - I therefore have no acceleration - therefore I have no weight according to your formula - yet the scale tells me different. How do you explain that?

Gravity (G) is generally accepted to have a universal consant in Newtonian physics and it's effect on masses is a function of their products and the inverse of the square of the distance between them.

I'm out of time and must go -

anyone feel free to correct me - my last Physics class was 12 years ago.

Later

JJJ

[Dust Devil]

you guys make my head hurt. I like the description clunckdriver used.

[mellow_pilot]

http://ourworld.compuserve.com/homepage ... weight.htm

Mass is commonly substituted for weight in everyday conversation. If someone wants to split hairs, let them. Just ask where to put the gravitational coefficient in your weight and balance calculations. (and ask if they want the force on the plane at sea level or FL400, cause it is different)

[beechy]

They are actually the same thing.....Weight = Mass*the aceleration due to gravity(9.8m/s^2).....by definition a force is a mass x and acceleration. So in fact weight is refered to as the force of gravity. More accurately it is the force created by gravity. But there is a definate and real differance between mass and weight.

Gravity itself is an acceleration resulting from the spinning of the earth. The circular motion requires a CENTRIPITAL force (twords the center) and that requires a mass and an acceleration as well...the acceleration resulting is what we call gravity. We are subject to this acceleration because we have mass and are located atop this spinning planet with in the gravitational range.

Why does the earth spin? that is another story for another time.

[TopperHarley]

JJJ,

Here is one link to the formula I provided: http://www.phy6.org/stargaze/Snewt2nd.htm.

Here is another link which is somewhat easier to follow: http://science.howstuffworks.com/fpte1.htm

You also stated, "If weight is in Kilograms and Mass is in Kilograms which I multiply by a constant with the units of meters per second squared - how can I possibly get a result of my weight on Earth in Kilograms?"

The problem is that weight is NOT measured in kilograms. KG is a measure of mass, not weight, but they are often used interchangeably. If you use the Weight= Mass*Gravity equation, your final answer should be in the units of "newtons." The problem is that this unit of measure is not really widely known. For interests sake, 1 newton= the force required to accelerate a mass of 1 kg at a rate of 1 m/s2. Here is a good explanation from "how stuff works:" http://science.howstuffworks.com/fpte2.htm

You also stated, "This brings up another fun point of discussion that touches on relativity and objects in motion - consider a closed system - me sitting here in front of my computer where I'm at rest and not falling - I therefore have no acceleration - therefore I have no weight according to your formula - yet the scale tells me different. How do you explain that?"

The fact is that the earth is still exerting its gravitational pull on you at the rate of 9.8m/s2. Even though you are in a state of equilibrium, this does not mean that this force disappears. It just means that it is being balanced by an equal and opposite force.

For instance, thrust itself is an example of an unbalanced force. When you consider drag along with thrust, however, there is no longer an imbalance. If a rocket is in orbit in outer space where there is no air density (and thus no drag), then if the astronauts activate the rockets, there will be no force to balance thrust. The rocket will simply keep accelerating.

I hope this helps. I know its technical in nature, so I kind of have a difficult time trying to explain it in a simple way.

[JigglyBus]

Beechy, beechy, beechy,

I love when someone tries to give an answer to a relatively simple question, and then states something like....

"Gravity itself is an acceleration resulting from the spinning of the earth"

Fish, the experts have spoken, so take everything you hear here as fact.

Wait, better idea, do the research yourself, you'll learn way more that way.

[Jet-stream]

Beechy,

I have a question for you, if the earth stops spinning, what happens to gravity?

[ODDERGUY]

JigglyBus!!!

Spoken like a true profesional!!!

My hats off to you sir!

Or mam

[beechy]

i will simplify.... Weight = force of gravity..... the guy in question at the beggining of this thread, who fish mentioned, is a boob who is talking about something he probably knows little about.

I was just trying to help.....

[weasello]

Gravity itself is an acceleration resulting from the spinning of the earth.

[heavymetal]

Gravity itself is an acceleration resulting from the spinning of the earth

Anything that has mass exerts a gravitational force. The reason everything sticks to the Earth is the fact that it's mass is far greater than anything else on it.

[Flood]

Gravity itself is an acceleration resulting from the spinning of the earth.

Now that's a pretty good one... Especially when you consider that the rotation of the earth would actually work AGAINST gravity... But what do I know...

[grimey]

If weight is in Kilograms and Mass is in Kilograms which I multiply by a constant with the units of meters per second squared - how can I possibly get a result of my weight on Earth in Kilograms.

Correct me if I'm wrong but perhaps you're thinking of the Force acting on a body (F) is equal to it's Mass (m) multiplied by acceleration (a) :
F=ma

Weight is a force, and is measured in pounds, newtons, etc., not kilograms. As far as how heavy someone is, 1 pound is equal to 454 grams only in earth's standard gravity. If you weigh 200 lbs on earth, you're roughly 90 kg in mass. On the moon, you'd still be 90 kg in mass, but you'd only weigh about 35 lbs.

[ragbagflyer]

Beechy, I hope for the sake of all passengers that you are not a pilot. I guess the maule that i fly must lift off the ground because if I get that prop spinning fast enough it makes enough gravity to somehow pull itself into the air. That makes sense right? I mean if the earth rotates once every 24 hours, and my prop spins at 2600 rpm that must be enough rotation (and therefore gravity) to overcome the earths. That's as long as I can keep my gyro's from pulling themselves together (because of all the gravity) and ripping the space time continuum.

[mellow_pilot]

Ya, the earth spinning actually does work against gravity. That's why they like to launch rockets in to orbit as close to the equator as possible, it takes less energy, therefore it's cheaper.

If there was an asteroid in space that wasn't spinning would it still have gravity? Hell yes!

[co-joe]

Aircraft can only generate lift to overcome gravity by injecting vast amounts of money, all other principles of flight are wrong!

Definitely there should be 5 forces affecting an aircraft in flight. Cash is crucial, maybe even more important than lift!


I guess in a round about way weight could be considered gravity, or rather a measure of the force of gravity. Gravity alone is a constant, based on the mass of the planet, whereas weight is a measure of the force of gravity on a particular object, based on its mass.

[kavupilot]

Weight is a force, and is measured in pounds, newtons, etc according to grimey...i thought by reading this that finally everybody had finally figured it out........grimey weight is not measured in pounds...maybe you are thinking of slugs? an english unit

[grimey]

Weight is a force, and is measured in pounds, newtons, etc according to grimey...i thought by reading this that finally everybody had finally figured it out........grimey weight is not measured in pounds...maybe you are thinking of slugs? an english unit

A slug is a unit of mass, same as the kilogram.

Weight: "The force that a mass m experiences due to gravity of another mass, "

http://scienceworld.wolfram.com/physics/Weight.html

Mass: "The quantity of "matter" contained in an object"

http://scienceworld.wolfram.com/physics/Mass.html

Yes, I actually do know what I'm talking about.