Is the force of gravity affected by an object's mass?

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    Gravity Moon
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The discussion revolves around misconceptions about gravity, particularly in relation to the Moon, sparked by a philosophy TA's apparent misunderstanding of basic physics. Participants express disbelief that a college-level educator could make such an error, emphasizing that all objects with mass exert gravitational force, including on the Moon. A series of hypothetical physics questions are posed, highlighting the confusion surrounding gravitational concepts and the importance of basic scientific literacy.The conversation also touches on the quality of science education in the U.S., with some arguing that mandatory science education often ends after 9th grade, leading to gaps in knowledge among college students. Participants share anecdotes about testing students' understanding of gravity and related concepts, revealing a concerning level of scientific ignorance. The need for a foundational understanding of physics is emphasized, as well as the potential for developing a questionnaire to assess scientific knowledge across various disciplines. Overall, the thread underscores the significance of basic scientific education and the implications of widespread misconceptions in society.
  • #51
Pengwuino said:
You should be required to know the answers to at least most of those questions to hold public office. Then again that might bring government to a standstill.
I couldn't find the link but I remember the Institute of Physics (in the UK) publishing a similar questionnaire given to physicists and a large number got some questions like "can antibiotics kill viruses" wrong.
 
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  • #52
jhooper3581 said:
Well, during my lifetime most people have said there's gravity presentable on the moon, so I thought it would be a common sense thing. Maybe I didn't prove it more throughly. And what do you mean by negative? Like: Is there no gravity on the moon?

Phrak said:
1) Is there gravity if you are freely falling?

I think most people could recite two "facts":

a) A person is lighter on the Moon because the Moon is smaller than the Earth and the force of gravity is weaker.

b) Astronauts in space are weightless (people have actually seen them be weightless on TV).

If they don't happen to remember the fact that they learned in school at that particular time, they do remember what they've seen on TV and apply that to the Moon (the Moon is in outer space, right?)

I think you could phrase the question differently and get a correct answer. "Why is a person lighter on the Moon than they are on Earth?", for example. Or, "How far did Alan Shepard's golf shot travel?*", for example.

If they don't understand why astronauts are "weightless", they'll become confused about what happens on the Moon. (I think it's more common to see astronauts on the Shuttle or Space Station than to see old videos of astronauts walking around on the Moon.)

*0 inches, 2 or 3 feet, 40 yards into a crater, and, on his fourth shot, about 200 yards (it's really hard to get a good golf stroke when you're wearing a suit so bulky that you can only hold the club with one hand). But I bet most people would answer "miles and miles", so maybe that wouldn't be a good question. In fact, referring to an old golf poem, maybe golfers wouldn't be the best people to ask about gravity on the Moon:

Richard Armour said:
First golfer on the moon is he,
Yet mad enough to pop.
Because of the lack of gravity,
The poor lad’s putt won't drop.
 
  • #53
BobG said:
I think most people could recite two "facts":

a) A person is lighter on the Moon because the Moon is smaller than the Earth and the force of gravity is weaker.
Since g = GM/R^2, if the only difference between the Moon and the Earth was that the Moon is smaller, (that is, R is smaller) then gravity would be stronger, not weaker, at the Moon's surface.

BobG said:
b) Astronauts in space are weightless (people have actually seen them be weightless on TV).
Since g = GM/R^2, they are not weightless, just apparently so. R is only a few percent greater in the shuttle, or the ISS, than it is at the surface of the Earth, so their weight is little changed. They only appear weightless because they are in orbit along with the vehicle.
 
  • #54
jimmysnyder said:
Since g = GM/R^2, if the only difference between the Moon and the Earth was that the Moon is smaller, (that is, R is smaller) then gravity would be stronger, not weaker, at the Moon's surface.


Since g = GM/R^2, they are not weightless, just apparently so. R is only a few percent greater in the shuttle, or the ISS, than it is at the surface of the Earth, so their weight is little changed. They only appear weightless because they are in orbit along with the vehicle.

Nicely countered.
 
  • #55
jimmysnyder said:
Since g = GM/R^2, if the only difference between the Moon and the Earth was that the Moon is smaller, (that is, R is smaller) then gravity would be stronger, not weaker, at the Moon's surface.

I stand corrected. I forgot - a toy car is smaller than a real semi truck, which is why a fly landing on a toy car won't fall off the car in spite of the fact that the Earth is larger than the toy car. :rolleyes:

(On the other hand, I now understand why the set of hot wheels I bought for my grandson cost so much to send through the mail.)

Okay, okay, "smaller" is vague terminology, but I know the average person isn't going to go into a comparison between the Moon's mass and it's radius since I doubt they know either.

And, if the densities are comparable (which they're not - the Moon is less dense), the volume, and hence the mass, is inversely proportional to the cube of the radius, meaning the force of gravity decreases in proportion to the radius when comparing two similar objects.(This is also why the force of gravity would actually decrease if you descended an elevator shaft all the way to the center of the Earth).

Since g = GM/R^2, they are not weightless, just apparently so. R is only a few percent greater in the shuttle, or the ISS, than it is at the surface of the Earth, so their weight is little changed. They only appear weightless because they are in orbit along with the vehicle.

That's what I meant. People don't understand the concept of free fall. It's the visual images they see without understanding that causes confusion when trying to visualize what happens on the Moon.
 
  • #56
DaveC426913 said:
Nicely countered.
I should point out that BobG had quotes around the words fact and weightless.

I just meant that the general public does not know enough physics to work out the problem. They just repeat what they are told even if they are told nonsense. Most can be taught, but it seems that the unteachable ones, or those with an anti-establishment message, are the noisiest. And so the nonsense propagates.
 
  • #57
BobG said:
a toy car is smaller than a real semi truck, which is why a fly landing on a toy car won't fall off the car in spite of the fact that the Earth is larger than the toy car.
Can you run this by me again? I don't understand what you mean. That is, besides the fact that flies don't even fall off the ceiling.

BobG said:
And, if the densities are comparable (which they're not - the Moon is less dense), the volume, and hence the mass, is inversely proportional to the cube of the radius, meaning the force of gravity decreases in proportion to the radius when comparing two similar objects.(This is also why the force of gravity would actually decrease if you descended an elevator shaft all the way to the center of the Earth).
Going below the surface of a planet brings about a different change. As you go below the surface M changes. The only M that counts is the stuff closer to the center than you are. Then as you (sort of) say, M is proportional to the cube of the radius, g decreases as you go down. However, this is irrelevant if you are comparing gravity at the surface of two different objects.
 
  • #58
BobG said:
I stand corrected. I forgot - a toy car is smaller than a real semi truck, which is why a fly landing on a toy car won't fall off the car in spite of the fact that the Earth is larger than the toy car. :rolleyes:

(On the other hand, I now understand why the set of hot wheels I bought for my grandson cost so much to send through the mail.)

Okay, okay, "smaller" is vague terminology, but I know the average person isn't going to go into a comparison between the Moon's mass and it's radius since I doubt they know either.

And, if the densities are comparable (which they're not - the Moon is less dense), the volume, and hence the mass, is inversely proportional to the cube of the radius, meaning the force of gravity decreases in proportion to the radius when comparing two similar objects.(This is also why the force of gravity would actually decrease if you descended an elevator shaft all the way to the center of the Earth).

jimmysnyder said:
Can you run this by me again? I don't understand what you mean. That is, besides the fact that flies don't even fall off the ceiling.


Going below the surface of a planet brings about a different change. As you go below the surface M changes. The only M that counts is the stuff closer to the center than you are. Then as you (sort of) say, M is proportional to the cube of the radius, g decreases as you go down. However, this is irrelevant if you are comparing gravity at the surface of two different objects.

If a toy truck had the same mass as a full size semi, the toy truck would have a larger force of gravity.

Besides, I think I get your point. The average person would instinctively realize a smaller planet has to have less mass and less gravity (just like a toy car must be lighter than a semi), but toss in one equation and there's a good chance for confusion (although completely ignoring the "proportional to the product of the masses" part is a pretty big omission even for the average person - even fifth graders understand gravity).
 
  • #59
BobG said:
If a toy truck had the same mass as a full size semi, the toy truck would have a larger force of gravity.

No, it would have a larger surface gravity. Hwever, at the same distance from the center of mass, both vehicles would have an identical pull.
 
  • #60
"I couldn't find the link but I remember the Institute of Physics (in the UK) publishing a similar questionnaire given to physicists and a large number got some questions like "can antibiotics kill viruses" wrong."

I have noticed that physicists in particular, and scientists more in general, are sometimes willfully ignorant of even the most basic principles of other scientific disciplines. The best reason I can come up with for this ignorance is that they feel that other disciplines just don't really matter.
 
  • #61
negitron said:
No, it would have a larger surface gravity. Hwever, at the same distance from the center of mass, both vehicles would have an identical pull.
This is an important point to remember the next time you are sucked into a black hole.
 
  • #62
What is the interest level in there being a questionnaire...
- that asks basic questions about a variety of disciplines (say, 10 questions per discipline for a smattering of disciplines)
- where the taker records the age, country of origin, field, and employment

Then we can see if any interesting patterns arise. For instance, it would seem obvious that a physicist will do better on the physics section than a biologist, but would the physicist do equally well across other disciplines, or what?

It could be interesting.
 
  • #63
It is an interesting idea. The challenge would be in creating the questions, especially if one wished them to be of equivalent difficulty in each discipline.
 
  • #64
Yeah, that's the trick. We'd almost need a team to come up with the questions, and if we all chip in, then who's left to take the test?
 
  • #65
AUMathTutor said:
What is the interest level in there being a questionnaire...
- that asks basic questions about a variety of disciplines (say, 10 questions per discipline for a smattering of disciplines)
- where the taker records the age, country of origin, field, and employment

Then we can see if any interesting patterns arise. For instance, it would seem obvious that a physicist will do better on the physics section than a biologist, but would the physicist do equally well across other disciplines, or what?

It could be interesting.

Ophiolite said:
It is an interesting idea. The challenge would be in creating the questions, especially if one wished them to be of equivalent difficulty in each discipline.

Have different "masters" of each topic write the 10 questions for each section, and have them use "medium-range" questions. And most important, have them consult with each other on the questions they're using lol

I think that would be a fantastic idea.. I'd love to take that quiz
 
  • #66
I'll get it started.
Math: 2 + 2 = ____ .
Physics: If you have two electrons and two neutrons, how many particles do you have altogether?
Chemistry: If you start with a molecule containing two atoms and you add another two atoms to it, how many atoms are in the new molecule?
Biology: If two horses have two foals, how many horses are there?
Astronomy: If there are two galaxies on the left of the photo image and two galaxies on the right, then how many galaxies are there all together?
Cosmology: Suppose there are two big bangs, and two big crunches. Then how many 'big' events are there?
Geography: If you visit two cities in the summer and two in the winter, then how many cities have you visited?
Philosophy: If two materialists are arguing with two idealists, how many philosophers are arguing?
Religion: If two angels can dance on the inner portion of the head of a pin and two can dance on the outer portion, then how many angels can dance on the head of a pin?

This should indicate who has a well rounded education.
 
  • #67
Jimmy is retired, by the way. In case you couldn't tell.
 
  • #68
This isn't physics, but I met a girl who thought the way unborn babies breathed was by breathing the "water" in the womb with their lungs. And apparently they inexplicably lose that ability once they're born.
 
  • #69
"The oxygen we breathe comes from plants."

Actually, half of it comes from algae. Some of it comes from cyanobacteria.
 
  • #70
ideasrule said:
"The oxygen we breathe comes from plants."
No it comes from stars, it just happens to pass through plants (or at least phytoplankton) ;-)
 
  • #71
jimmysnyder said:
I'll get it started.
Math: 2 + 2 = ____ .
How can I possibly answer this with only scalar values? 2i + 2j will give a distinctly different answer than (.5i+.866j+1.732k) + (1.5i+.886j+k)

Another math problem:

If a class has 20 boys and 20 girls, how many students are in the class?
If a class has 20 boys and girls, how many students are in the class?

This was actually a subject for discussion during a higher headquarters inspection of a squadron's evaluation scenarios. They initially claimed the regulation requirement for at least three ground system and satellite system malfunctions per scenario meant three of each.

To their credit, they eventually realized how stupid that sounded - they weren't very good at vector math. I explained how it took two of each to equal a total of three ... at which point I was kicked out and higher headquarters decided the squadron was doing things just fine.
 
  • #72
BobG said:
This was actually a subject for discussion during a higher headquarters inspection of a squadron's evaluation scenarios. They initially claimed the regulation requirement for at least three ground system and satellite system malfunctions per scenario meant three of each.
Similair thing happened with 3G phone band license auction.
The auction rules said that bids had to be multiples of £1000, one bidder didn't have any competitors in their region but bid £2000 because the lawyers weren't convinced that £1000 was a multiple of £1000 !
 
  • #73
D H said:


Note that the people inside the moonbase not equipped with those special heavy boots just float. The end scene shows that the heavy boot effect has limited applicability. Slap someone in the back and off he goes, only to get hit by a meteor streaking through the moon's thick atmosphere. Documented!


I couldn't help noticing, that when youtube users comment like this:

well, at least I'm not the only one who had a scientific issue with this commercial.. ooh, check out the bad continuity.. they're inside - floating - yet things like nametags and ties will hang like they're in normal Earth gravity.. yup.. I'm a geek.. lol

Why are people walking on the surface, but floating indoors? How does that work? There's gravity on the moon, but only outdoors?

they get voted thumb down as dumb comments, but when somebody responds like this:

the asteroid thing wouldn't work and fedex doesn't ship to the moon, i don't think theyre going for accuracy

it gets voted thumb up as good explanation why one shouldn't get stuck with little scientific inaccuracies.

It could be that the collection of people who voted in this isn't very large, but caught my attention anyway.
 
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  • #74
DaveC426913 said:
Jimmy is retired, by the way. In case you couldn't tell.
Sadly no, I'm still working on my Master's thesis in Mathematics: The effect of a non-closed commutative arithmetic operator on a certain subset of the even integers. I already have a result. With proper care in choosing the subset, you can relax the condition that the operator be commutative.
 
  • #75
Jimmy is single and has no social life by the way. In case you couldn't tell.
 
  • #76
DaveC426913 said:
Jimmy is single and has no social life by the way. In case you couldn't tell.

Why are we talking about Jimmy? Is this some running gag that I'm not getting?
 
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  • #77
ideasrule said:
Why are we talking about Jimmy? Is this some running gag that I'm not getting?
Jimmy's not threatened by ideasrule's question. Jimmy's happy for ideasrule.
 
  • #78
ideasrule said:
Why are we talking about Jimmy? Is this some running gag that I'm not getting?
:biggrin:Jimmy spent a lot of time writing that post (#66) is all.
 
  • #79
DaveC426913 said:
Jimmy is single and has no social life by the way. In case you couldn't tell.

Jimmy's wife told you that, didn't she.
 
  • #80
D H said:


Note that the people inside the moonbase not equipped with those special heavy boots just float. The end scene shows that the heavy boot effect has limited applicability. Slap someone in the back and off he goes, only to get hit by a meteor streaking through the moon's thick atmosphere. Documented!


Easy Peasy. Thems magnetic boots.
 
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  • #81
jimmysnyder said:
I'll get it started.
Math: 2 + 2 = ____ .

I remember reading here on PF a couple of years ago, that sometimes 2+2=5, for large values of two.
 
  • #82
Chi Meson said:
I remember reading here on PF a couple of years ago, that sometimes 2+2=5, for large values of two.

Yes, Jimmy has laboured long and hard on errata page...
http://www.erratapage.com/
 
  • #83
jimmysnyder said:
Math: 2 + 2 = ____ .

Answer: 1.
jimmysnyder said:
Cosmology: Suppose there are two big bangs, and two big crunches. Then how many 'big' events are there?

Answer: 2.
 
  • #84
The real idiots are those who believe the moon landing happened...
 
  • #85
JasonRox said:
The real idiots are those who believe the moon landing happened...

...in a film studio?
 
  • #86
ideasrule said:
...in a film studio?

It was filmed in a desert. Duh.
 
  • #87
Oh God, can we not start this masturbatory science vs. religion thing again on these physics forums. Amen.
 
  • #88
JasonRox said:
It was filmed in a desert. Duh.
On Mars


[ps removed Arizona senator link to keep AUMathTutor happy]
 
  • #89
AUMathTutor said:
Oh God, can we not start this masturbatory science vs. religion thing again on these physics forums. Amen.

What does Moon hoaxing have to do with science versus religion? :confused:
 
  • #90
JasonRox said:
The real idiots are those who believe the moon landing happened...

Haven't there been a few moon landings?
 
  • #91
leroyjenkens said:
Haven't there been a few moon landings?
Let's just keep things real here.

JasonRox and everyone else is just joking around about faked Moon landings. You won't find any serious Moon hoaxers in these parts. At least, not for long.
 
  • #92
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  • #93
SHOW THAT lo^3 is the volume of the cube,then lo^3*(1-v^2/c^2)^.5 is the volume viewed from a reference frame moving with uniform velocity V parallel to an edge of the cube.
 
  • #94
What does this have to do with the Moon landings?

Just use the length contraction formula to figure out the dimensions of the cube in the moving reference frame. Also, homework questions should be posted in the "Homework and Coursework" forum, not in General Discussion.
 
  • #95
Reminds me of this video



Conversion factor fail
 
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  • #96
Eigenslam said:
Reminds me of this video



Conversion factor fail


I think if you get them to acknowledge that .5 dollars is half a dollar and .5 cents is half a cent, they would be left with no option but to acknowledge .002 dollars and .002 cents aren't the same.
 
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  • #97
im not even going to take the time to read through the statements and questions, Yes, their is gravity on the moon, and that TA is the stupidest person ever. That may be politically incorrect, however its the dramatic effect intended :P
 
  • #99
Ok, an ignorant question: I understand that Newton's laws tell us that in a vacuum, two objects will fall at the same rate, regardless of mass.

But relativity says that gravity is the warping of space by mass. It would seem intuitive that something with more mass would cause more curving of space, hence greater gravitational "attraction."

Why is this not the case?
 
  • #100
Um, it is the case. If Earth were more massive, it would pull on objects with more force.
 

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