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Feedback Comments on Gravity

by Ron Kurtus

A total of 78 comments and questions have been sent in on Gravity. They are listed according to date.

You can read them to further your understanding of the subject.

List of next 10 letters




Overview of Gravity Is the definition of gravity correct? K.S.A
Artificial Gravity Science fiction on artifical gravity USA
Overview of Gravity Is gravity why we don't fly off into space? USA
Work Against Gravity and Inertia Wants formula to lift with a motor India
Equivalence Principle of Gravity Confused about gravitation with a small object USA
Gravity I think that the calculations are wrong USA
Gravity How does gravity affect the human body? USA
Gravity Is there a point in the Universe with zero gravitation? USA
Gravity A force to counter gravity in aircraft flight UK
Gravity Should use SI units USA

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First 10 letters

Is the definition of gravity correct?

Topic: Overview of Gravity


December 20, 2012

I was wondering, we learned that gravity is the force that pulls all objects together. And basically, we are pulled more to the Earth than to each other because the Earth just has an extremely bigger mass, that our attraction to each other, then seems negligible. You, though, defined gravity as "the force that pulls us to the planets". How true is this theory, you think, that gravity pulls us all towards each other? I mean, how trusted can it be since it depends on the big bang theory? What are your thoughts, since you, yourself, chose to ignore that theory in your definition?

Thank you.

P.S. I realize this was just an "overview", but still your neglecting that theory made me wonder.

- K.S.A



There is a difference between gravity and gravitation that is often not explained in science courses. Sometimes the expression "gravity" is carelessly used for both terms.

Gravitation is the force of attraction between objects. It is most often seen in the attraction of large objects in space, although it even exists between small objects.

Gravity, on the other hand, is the attraction of object toward the Earth. There is also gravity of the Moon and gravity of Mars, and so on. Gravity is an approximation of gravitation for small objects near a large one, such as the Earth. See Overview of Gravitation for more information.

The Big Bang Theory concerned gravitational forces.

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Science fiction on artifical gravity

Topic: Artificial Gravity


November 12, 2012

Greetings, I am a science fiction writer, and I'd like to postulate a hypothesis for your notions and feedback. If one were to create a form of crankshaft that spins energy through magnetic coils in a spiraling circuit, allowing for hyper-acceleration of particles for the production of anti-matter, similar to what is used in super-colliders, and this tube structure itself were to be spinning, further increasing the momentum of the charged particles generated within, could this spinning collision device be used to both produce anti-matter and generate artificial gravity if used in the core of a large space ship? Furthermore, this anti-matter could be funneled to one end of the cylinder and recombined with positive matter for propulsion. If this hypothesis were to hold true, would it not reduce the amount of mass needed to generate gravity, and allow for better streamlining of a space vessel, since it would also be generating fuel for propulsion?
Alternatively, if a circular super-collider were to be used for the outer rim of a disc-shaped vessel, could it's function in accelerating matter at beyond light speed alone create the momentum needed for artificial gravity, while again producing anti-matter which could be used as fuel for propulsion?




You've got some complex ideas.

For some reason, most matter in the Universe is right-handed and has certain charges. Anti-matter is simply a particle with the opposite charge and perhaps spins in a left-handed direction. Radioactive decay and high energy collisions can result in antiparticles being emitted.

See Antimatter and Antiparticles.

Also so-called artificial gravity is simply caused by inertia. It has been used many times in science fiction.

Some better topics to consider are dark matter and dark energy. They are only theory but explain many things in the Universe. It is a good topic for science fiction, since you can almost create your own special properties from these items. For example, if someone could convert to dark matter, he would be invisible to normal people.

Best wishes in your writing. Keep in touch on your progress.

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Is gravity why we don't fly off into space?

Topic: Overview of Gravity


October 30, 2012

i was just thinking gravity is the reason we do not fall from the earth, as does not water and all objects on this planet..its weird but plausible....

judy - USA



Yes, gravity is what pulls everything toward the Earth. The rotation of the Earth causes a centrifugal force that would cause objects to fly off into space, but it is too small to overcome the force of gravity. Thus, we are held down.

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Wants formula to lift with a motor

Topic: Work Against Gravity and Inertia


October 27, 2012


ADITYA - India



I'm sorry, but we do not have that information.

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Confused about gravitation with a small object

Topic: Equivalence Principle of Gravity


July 26, 2012

I am having difficulty with this concept.

Regarding falling bodies it is my understanding that
F = m1*a = G*m1*me/r^2 --> a = G*me/r^2 for object 1,
'me' being the mass of the earth.

As both objects are actually accelerating the earth's acceleration would be based on
F = me*a = G*m1*me/r^2 --> a = G*m1/r^2 for the earth.

For a 1 kg object at 1 kilometer and estimating G at 7*10^-11 N(Kg/m)^2, the earth's acceleration would be

a = (7*10^-11)(1)(10^-6)= 7*10^-17.

For a 2 kg object it would be 14 * 10^-17.

So small a difference as to be the same for practical purposes? Sure, until the mass of the other object approaches that of the earth sized object. I don't mean to be thick headed, but I could really use some help here. People regularly mention cancelling out the earth's gravitation when viewing it from the perspective that the other object is accelerating, while ignoring the perspective that the earth is accelerating. Its only in the past century and a half (speaking somewhat loosely) that we've been able to measure such small differences. If you could point me at some modern experiments I would be overjoyed.

Thanks for your time.

Alex - USA



At distances relatively close the the Earth, Newton's Universal Gravitation equation F = GMm/R^2 can be approximated as F = mg. Gravity Constant Factors has a chart showing that at distances above 6 km from the Earth's surface, the error in using F = mg instead of Newton's equations is less that 0.1%.

When dealing with small objects relative to the mass of the Earth and at distances relatively close to the surface of the Earth, the gravity equation is used. When dealing with large distances and astronomical bodies, you use the gravitation equations. That is why the section has been divided into those two areas.

Another factor in gravitation calculations concerns the center of mass between two objects.

I hope this helps your understanding.

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I think that the calculations are wrong

Topic: Gravity


March 15, 2012

I think that the calculations are wrong if a meter is 3.3 feet then the 32.2 fps2 is wrong it should be 32.3631 and if you wanted to round up like they said they did then 32.3631 would be 32.4 not 32.2 so I was just wondering would that change the 9.8 newtons that g= in f=mg

Edward - USA



Actually, 1 m = 3.28 ft. See our English-to-Metric tool link on each page.

If we start with g = 9.807 m/s^2 from Overview of the Force of Gravity and multiply 9.807 X 3.28 = 32.167 ft/s^2. That rounds out to 32.2 ft/s^2.

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How does gravity affect the human body?

Topic: Gravity


February 26, 2012

I'm interested in how gravity affects the human body; standing erect, bending forward, leaning to one side. Are there equations for these angles relating to the gravitational force on the human body?

Gay - USA



Gravity pulls all the atoms in the body toward the ground. You can consider the human body a solid object with a center of gravity (CG) according to the orientation and configuration of the body. See Center of Gravity.

If a person was bending over, there would be a tendency of the body to tip over, rotating about the fixed point where the feet are on the ground. What happens is that your muscles change the angle slightly and stabilize the body to prevent it from falling over.

There are no real equations to use. It is mainly done by geometric drawings, similar to those in the lesson.

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Is there a point in the Universe with zero gravitation?

Topic: Gravity


February 22, 2012

In advance, Please forgive me for my spelling, grammer and rambling on.

Is there any such space in the known universe where there is zero gravity (meaning absolutely no gravitational pull or effect)all the time?

My understanding is that all MATTER has some form of gravitational pull. Also, all MATTER in the universe is in motion.

Therefore, is everything in the universe effected by gravity and have an effect on everything else?

Example; If a major star, planet or object was suddenly erased completely, would the entire universe be effected?

One theory I have is that the entire universe is like an "ecosystem". Meaning in some way it is all connected and self-dependant on everything in it. Any change will have some effect on the rest of it; big or small.

I feel this supports inteligent design. The Universe was Created and set in motion on purpose, and for a purpose. There can not possibly be this many "coincidences" coming so perfectly togeather.

I really would love and appreciate your views on this?

Thank you,
John Blackinton

John - USA



If the Big Bang Theory that matter in the Universe exploded from some center point is correct, I would imagine that point could have zero net gravitational pull on it, since matter is probably equally distributed around that point.

A good way of thinking about it is like a form of ecosystem.

As far as supporting Intelligent Design, see Big Bang Theory and Religion.

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A force to counter gravity in aircraft flight

Topic: Gravity


February 9, 2012

I am looking at the principles of flight, particularly lift. Given the aircraft has thrust that contributes to lift in a climb, some of the weight is catered for by this thrust (other than aeodynamic - wing - lift). Now in straight and level flight the plane is effectively climbing (must have angle of attack). I am concerned, though I agree with the content here on gravity, that another force than lift exists to counteract gravity. With a plane we are talking not about a solid streamlined object, like a bullet, but something that has vertical wind resistance, which you will agree, changes the rate of descent. Also the plane has power constantly applied, unlike the bullet. With this powered forwards motion is the plane effectively seen to be lighter than it would be on the ground (ignoring inverse square law etc due to height above C of G)? It seems I am talking in the time domain, where a vehicle crossing a weak bridge at speed might get across, where at slow speed the bridge would collapse. Thanks for reading my question.

Michael - UK



Gravity can be considered a constant force toward the ground. For relatively short distances, it is straight down. But if the object is moving in a straight line. over a great distance, the curvature of the Earth must be taken into account. The inertia of the flying object make it want to go in a straight line, which is counter to gravity when the curvature is taken into consideration.

See Gravity and Newton's Cannon. As the cannonball moves faster, it will soon travel in a line into space. If the ball or object were continually propelled, the effect would be more dramatic.

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Should use SI units

Topic: Gravity


February 7, 2012

This is a great resource but it would be more useful in SI units since that's the standard for science nowadays.

Brendan - USA



Since so many non-scientific people in the U.S. still use the English system of measurement, I've tried to use both English and SI in all the Gravity lessons. Some example problems are in feet/sec, while others are in meters/sec.

However, in the Gravitation section, only the SI units are used.

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