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

[jhooper3581]

Is there gravity on the moon?

Well of course there is. If you would ask any person walking down the street, they would most likely say yes. It's just that Earth's gravity is a little bit more stronger, hence more weight is presentable, but same mass as it is on the moon.

 

[Phrak]

Is there gravity on the moon?

Well of course there is. If you would ask any person walking down the street, they would most likely say yes. It's just that Earth's gravity is a little bit more stronger, hence more weight is presentable, but same mass as it is on the moon.

Are you sure it's not an ill posed question. If so, how could you answer to the negative?

 

[jhooper3581]

Are you sure it's not an ill posed question. If so, how could you answer to the negative?

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]

Question #2
Does the Earth go around the Sun, or does the Sun go around the Earth?

I understand the rules about linking to unvetted information sources, but in this case does the National Science Foundation know best or https://www.physicsforums.com/showthread.php?t=245334"?

Did you think this was a crackpot link......^....above. Or maybe something like the Physics Forum?

 

[Phrak]

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?

1) Is there gravity if you are freely falling?

2) What is gravity? Does it mean a force like gravitational attraction or are they speaking of the general phenomena of gravity, where one such phenomenon is gravitational attraction.

I should admit that it could be a well posed question for 12th graders, but 'gravity' is a not a good replacement for 'gravtitional attraction' when speaking to physicists.

 

[jhooper3581]

1) Is there gravity if you are freely falling?

2) What is gravity? Does it mean a force like gravitational attraction or are they speaking of the general phenomena of gravity, where one such phenomenon is gravitational attraction.

I should admit that it could be a well posed question for 12th graders, but 'gravity' is a not a good replacement for 'gravtitional attraction' when speaking to physicists.

Well, I've just graduated from high school not too long ago, and I took basic physics class in junior year. For the first question, if you are free falling, then isn't there still gravity left, because someone can determine the speed of falling, due to gravity. I don't know haha

 

[cristo]

Editorially, it is unfortunate, but mandatory science education in the US basically ends after 9th grade, iirc. Kids looking toward a scientific background take science classes in high school, but they aren't required.

Really? I just presumed that students had to take basic classes in all subjects during high school. Over here, the bare minimum that you have to take until the end of compulsory schooling (at 16) are english (lit & lang), maths and science. Do US students get to take whatever they want? This is presumably the reason why US colleges have the general education courses that students have to take. It seems pretty backwards to me!

 

[AUMathTutor]

That's refreshing to see that US respondents were scientifically competitive with their European counterparts.

It looks like the US was a little weaker in the life sciences, and a little stronger in the physical sciences.

I wonder what a comparison of mathematical knowledge would demonstrate.

 

[Office_Shredder]

I know the people polled were college students: they should be more than qualified to answer this question. Editorially, it is unfortunate, but mandatory science education in the US basically ends after 9th grade, iirc. Kids looking toward a scientific background take science classes in high school, but they aren't required. So a week or two about Newtonian physics in 8th grade might be all an American student ever sees.

I don't think you recall correctly. Most high schools if not all will require a year of physics chemistry and biology or something similar (required by the state the high school is in)

 

[CRGreathouse]

In my state high school students are required to take a year of chemistry, a year of physics, and a year of biology (6 semesters), along with four years of math (8 semesters).

 

[matthyaouw]

I don't buy this story. It was convincing 'til here:

To prove my point, we went back to our dorm room and began randomly selecting names from the campus phone book. We called about 30 people and asked each this question: 1

1. If you're standing on the Moon holding a pen, and you let go, will it
a) float away,
b) float where it is,
or c) fall to the ground?

About 47 percent got this question correct.

Call strangers at random with a question like this, and at leat 80% will answer "WHAT!? I'm on the no-call list. Call me again and I'll SUE!"

 

[TheStatutoryApe]

In my state high school students are required to take a year of chemistry, a year of physics, and a year of biology (6 semesters), along with four years of math (8 semesters).

When I was in high school, a little over a decade ago, we were only required two years of science (physics and biology) and three years of math.

 

[Astronuc]

In my state high school students are required to take a year of chemistry, a year of physics, and a year of biology (6 semesters), along with four years of math (8 semesters).

That was the requirement when I went to high school in Texas during the 1970's. Biology (10th grade), Chemistry (11th grade), and Physics (11th or 12th) grade. There was a general science course in 9th grade, which was more or less introductory physics/chemistry with a broad basis. Mathematics was taught every year from 7th through 12th grade, but 7th and 8th were fairly redundant.

9th grade was Algebra I
10th grade Geometry
11th grade Algebra II (some analytical geometry)
12th grade Analytical Geometry and Calculus

My 10th grade honors Geometry course was taught in one semester, so we the Trignometry course during the second semester. Unfortunately, our 10th math teacher burned herself out (exhaustion) by the end of the first semester, and we got a different teacher who wasn't quite as intense as the first teacher.


I'm pretty sure I learned about tides as early as 6th grade. Tides are affected by Sun AND Moon by virtue of gravity.

Of course, all education is not equal, and all students are not equal.

 

[Jimmy Snyder]

I don't buy this story.

Me either. Too many people know about that hammer vs feather video for this story to be believable.

 

[D H]

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!

 

[mgb_phys]

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.

 

[BobG]

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?

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:

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.

 

[Jimmy Snyder]

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.

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.

 

[DaveC426913]

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.

 

[BobG]

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.

(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.

 

[Jimmy Snyder]

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.

 

[Jimmy Snyder]

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.

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.

 

[BobG]

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.

(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).

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).

 

[negitron]

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.

 

[AUMathTutor]

"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.

 

[Jimmy Snyder]

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.

 

[AUMathTutor]

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]

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.

 

[AUMathTutor]

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?

 

[Kronos5253]

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.

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

 

[Jimmy Snyder]

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.

 

[DaveC426913]

Jimmy is retired, by the way. In case you couldn't tell.

 

[leroyjenkens]

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.

 

[ideasrule]

"The oxygen we breathe comes from plants."

Actually, half of it comes from algae. Some of it comes from cyanobacteria.

 

[mgb_phys]

"The oxygen we breathe comes from plants."

No it comes from stars, it just happens to pass through plants (or at least phytoplankton) ;-)