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# Force and Weight

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OK, Force: F=ma

OK, Weight: W=mg

And of course, there exists that conversion between Newtons and pounds, which can be related in the following analogy: newtons: Pounds::farenheit:celcius.

Is weightactually a "force" oram I missing something here? (take in mind I'm onlyin High School).

Thanks,

Moonie

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Yes, weight is a force.

But it's not one of the fundamental forces of nature: it's the result of one (gravity). It's like you hitting a baseball with a bat: the force the bat applied to the ball is clearly a force, but it's not one of the fundamental forces (it is, however, the result of one of the fundamental forces - the electromagnetic force at the atomic scale).

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Anything with a mass has force, right?

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TeleMad, I would beg to differ. weight is NOT a force. weight is a MEASUREMENT of the force of gravity exerted on a small object by a very massive one.

and the baseball/bat analogy is a poor one. the forces at work here has little to do with weight (and the strong nuclear force is a bit too complicated for your analogy to be realevant on this macro scale when there are other forces to better explain the differences between weight and forces). They do however, employ the principles of potential energy, and kinetic energy and mass. Potential energy in the bat being held. kinetic energy when the bat is swung. kinetic energy as the balls travels to the bat. When the bat and ball make contact, the transfer of kinetic energy from the bat to the ball is far greater than that of the ball to the bat due to the vast differences in mass between the ball and the bat being swung by the hitter.

Aki, you are correct in a sense... all things that have mass either have potential or kinetic energy.

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deamonstar: (and the strong nuclear force is a bit too complicated for your analogy to be realevant on this macro scale ...

You are confused. I said NOTHING about the strong nuclear force. Read it again.

deamonstar: They do however, employ the principles of potential energy, and kinetic energy and mass. Potential energy in the bat being held.

Nope. In regards to hitting a ball, a bat being held doesn't have any potential energy. It’s not elastic and its shape is not distorted in any way such that it stores energy that would cause its shape to be restored (as a spring, a rubber band, or a bow would), and its gravitational potential energy is completely irrelevant to hitting a ball.

deamonstar: Aki, you are correct in a sense... all things that have mass either have potential or kinetic energy.

Not in the normal sense. A rock sitting motionless on the ground has mass but has neither kinetic nor potential energy.

deamonstar: TeleMad, I would beg to differ. weight is NOT a force. weight is a MEASUREMENT of the force of gravity exerted on a small object by a very massive one.

You finally got one right! :-) Yeah, I goofed there.

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an object would appear very slightly lighter when its higher in the sky, is that right?

greater distant, lease gravity...

hmm, im wondering....

relativity to an object less mass than earth, earth's weight would be the same no matter what...?? because the object is falling to earth, not the earth falling to the object...?

im just wondering that the acceleration of an object due to the earth is unchanged by the mass of that object... but distant does matter...whatever... :S

confusing..

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Originally posted by: Tim_Lou

an object would appear very slightly lighter when its higher in the sky, is that right?

greater distant, lease gravity...

Technically, yes. Eventually the object becomes weightless if it is far enough away (or goes into orbit). However, it does not become massless because it will always be attracted to *something*. Say, if you shoot a satellite at the moon it will require less and less fuel to power the rocket but it does not escape the gravity of the earth...*until* it is captured by the moon.

relativity to an object less mass than earth, earth's weight would be the same no matter what...?? because the object is falling to earth, not the earth falling to the object...?

If I understand gravity right, then both objects attract each other. However, the Earth is so much more massive than anything that is falling towards it that the effect on the Earth is negligible.

However, if a large object, like a brown dwarf, passes through our solar system then the Earth would feel the effect and might shoot out of orbit and leave the solar system entirely.

im just wondering that the acceleration of an object due to the earth is unchanged by the mass of that object... but distant does matter...whatever... :S

Newton proved that mass is not relevant to the speed of accelleration. However, as i mentioned above, relative mass is important to the attraction the bodies have upon each other.

Gravity is not the same as magnetism, you know.

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"mass is not relevant to the speed of accelleration"

mass is relevant to gravity.

gravity is relevant to acceleration.

but mass is not relevant to acceleration??? i know its true... but its just plain confusing...

although i understand that fact the gravity is a curve of space, falling due to this curve is a constant if having a constant distant....

the problem is that i dont understand y gravity doesnt add up.... the curve does not add up to a "deeper" curve, the acceleration remains the same... the attraction force of the object to the earth is irrelevant...

lets say that one object is more massive than the earth, now the acceleration changes, b/c its not the object fallin to the earth anymore, but the earth falling to that object...

but what about if they have the same mass?

and what happen as the mass of one object approaches to the mass of earth, there is a sudden change of acceleration??? that would mean in the graph of acceleration, there is a discontinuity... which isnt the natural of universe...

im just plain confused : (

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Yeah, I've always wondered what would happen if a planet identical to the Earth, ie with the same mass and gravitational field, approaches the earth. What would happen to us, and all the things on the Earth, would we all start floating around, because gravity is "cancelled"?

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Originally posted by: Tim_Lou

gravity is relevant to acceleration.

but mass is not relevant to acceleration???

Gravity is relevant to acceleration - and MASS is relevant to acceleration. What I wrote was that mass is not relevant to the SPEED of acceleration. But this is not completely true, I oversimplified and it came out wrong. Sorry.

What decides the rate of acceleration is the mass of a body and the gravitational pull it has on another object, plus the pull which the other object has on the first object.

In essense, "gravity is just the force per unit mass exerted on one body by another" (see source below). However, the gravitation constant, G, is universal. It is the mass of the very large object and the relative acceleration of objects falling towards it we are talking about.

So on Earth the acceleration due to gravity is g=9,8m/s^2, because of the mass of the Earth. On the moon g=1,62m/s^2. (http://hypertextbook.com/facts/2004/MichaelRobbins.shtml)

Here is a very good source about gravity: http://scienceworld.wolfram.com/physics/Gravity.html

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Originally posted by: Aki

Yeah, I've always wondered what would happen if a planet identical to the Earth, ie with the same mass and gravitational field, approaches the earth. What would happen to us, and all the things on the Earth, would we all start floating around, because gravity is "cancelled"?

No, gravity would not be cancelled. It would bring the earth out of stable orbit, however. People on the earth would not feel the gravity of the other object, but the earth would. If the object crashed into the earth then we would be in serious trouble... But most likely the earth would either move into a new stable orbit (closer to or further out from the sun), fall into the sun, or start moving away from it.

The sun's pull on the earth is so much greater than what a planet would have so the earth would still feel the pull of the earth as a stronger force. Gravity is after all a very *weak* force (for example, the mass of the earth compared to the mass of a person is extremely different, yet we can walk around without problems).

I hope someone with a better physics insight than me can correct me or elaborate.

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Tim_Lou

"mass is not relevant to the speed of accelleration"

mass is relevant to gravity.

gravity is relevant to acceleration.

but mass is not relevant to acceleration??? i know its true... but its just plain confusing...

That also perplexes me...

if F=G(m1*m2)/r^2 where (m1 and m2 are masses of 2 objects and r=distance between the objects and G is the universal gravitational consant)

how is it that mass is not relevant to the acceleration?

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alexander, I explained that on the previous page.

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Sorry Tormod, its just that it doesnt make too much sense, but in a sense it does. I read the post about 10 more times and i think the only explanation to my perplexion is: "Thats the way it is, so deal with it" and i will. Maybe its just something that I'm missing, something that will hold the key for my understanding of the phenomenon and is the reason for my nonunderstanding of gravity.

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Yeah, I've always wondered what would happen if a planet identical to the Earth, ie with the same mass and gravitational field, approaches the earth. What would happen to us, and all the things on the Earth, would we all start floating around, because gravity is "cancelled"?

No Gravity will not be cancelled; the electrical force is for example both attractive andrepulsive; the gravitational force is only attractive. if a massive object would approuch earth... well you would get massive tidal waves (remember: the tides are caused by the gravitational attraction of the moon). Maybe some vulcanic explosions on the side where the planet approaches; and after a while yoiu would be in serious trouble

More interesting is perhaps the inclusion of a 2nd 'sun'; directly opposit, wrt the earth with our first sun.

The attraction by those suns would then by equal, but in opposite direction. (this is basicly equal to 'removing the sun') so the net force is 0.

What would happen then? Well the motion of the earth would be determined by its current speed and the gravitational attraction of the other planets... wild guess: We would move in a somewhat wobbling line out of the ex- solar system....

mass is not relevant to the speed of accelleration"

mass is relevant to gravity.

gravity is relevant to acceleration.

but mass is not relevant to acceleration??? i know its true... but its just plain confusing...

As tormod sayd: mass is relevant to accelaration. Since for any force we have: Force=Mass*accelaration.

gravity is a special case, since the strength of the force is proportional to the mass. Basicly: GravForce~Mass1*Mass2. (~ means 'is proportional to'; so any constants are ommitted)

Now the nice thing is; inserting this in newtons formula and assuming Mass1<<Mass2 (so mass2 is e.g. the sun and mass1 is the earth). Mass1*Mass2~Mass1*Acceleration1 --> Accelaration1~Mass2.

So for the gravitational force, the net effect is that the accelaration of a particle is completely determined by the mass of the particle which CAUSES the attraction.

i hope this clarifies the point a bit...

Bo

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maybe the problem is that i dont understand y F=ma... isnt that the object falling on earth is also attracting the earth very very slightly???...

or relativity to the object, the earth is "falling" to the object?

so, F= m(of earth)*a ????

also, what about the theory of relativity, it describes gravity as a curve rather than a force..... ????

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maybe the problem is that i dont understand y F=ma... isnt that the object falling on earth is also attracting the earth very very slightly???...

Owkee... you misunderstand this a bit

you can take F=m*a actually as a definition of force; ALL forces obey this law. so this formula is not the formula that states where the force comes from, but how a particle will react when a force (any force) is applied to it.

The formula states: "If we apply a Force F to a particle with mass m, it will get an accelaration of F/m"

notice that there is NO interaction with other masses here!

the gravitational formula F_grav~M1*M2 states that "2 masses M1 and M2 attract each other with a force proportional to M1*M2"

So lets look at the earth-sun system. From the gravitational formula we have: F_grav~M_sun*M_earth

So now lets look what kind of accelaration this gives to the earth: we had: a = F/m. Since we're looking at the effect of F_grav on M_earth; we substitute those: a ~ F_grav/M_earth = (M_sun*M_earth)/M_earth = M_sun.

So the earth gets an accelaration proportional to the mass of the sun. Vice versa one can calculate that sun gets an accelaration proportional to the mass of the earth. Since the mass of the sun is much larger then that of the earth so the earth circles around the sun and not vice versa!

Bo