Gravity Question

[Kimpatsu]

I have been challenged with the followinng question by a fundy: "If the mass of the moon is low - about eighty times less than the earth, why is the gravity so high (about one sixth that of Earth's)?"

 

[Matabiri]

It's smaller in size as well as mass.

Gravitational acceleration is given by

(G m1)/(r^2)

where G is the universal gravitational constant (= 6.67300 × 10^-11 m3 kg-1 s-2), m1 is the mass of the body (= 5.9742 × 10^24 kg for the Earth, 7.35_x_10^22 kg for the moon), and r is the distance from the body (= 6378.1 km at the Earth's surface, 1737.4 km at the moon's). So although the Earth is 81 times the mass of the moon, because it's much larger the acceleration is only 6 times greater.

Edit for formatting

 

[RamblingOnwards]

Because the radius is so much smaller. See here for a description for laymen:

http://curriculum.calstatela.edu/courses/builders/lessons/less/les1/gravity.html

 

[epepke]

I have been challenged with the followinng question by a fundy: "If the mass of the moon is low - about eighty times less than the earth, why is the gravity so high (about one sixth that of Earth's)?"

(sigh) Because it's smaller.

[edited to add] Take 1/80 as the number. Assume similar density. Take the cube root of 1/80 to find the factor of radius. Square that to find the effect on gravity at the surface. Divide this into 1/80. It comes out to about 1/5, which is a bit high. But also, the density of the Earth is a bit higher than the density of Luna.

 

[wollery]

Gravity is a myth.....






















































The Earth sucks!



I'm sorry, it's been a very trying day.

 

[Brown]

How in the world would a perceived disparity support a fundamentalist position?

 

[CurtC]

A quick answer is that for planets of similar density, the gravity increases linearly with the size. So the Earth is 4x the size, so you should expect around 4x the gravity. That's a first-pass approximation, then you have to apply fudge factors, such as the Earth's higher density (is it because we have a big iron core?).

If he's curious, you can explain that with similar density, the mass increases with r³, and the gravity versus distance goes down by r², so those cancel out, and gravity increases linearly with r.

But I too have to know whether the question has to do with his being a fundy.

 

[69dodge]

Gravity weakens with increasing distance. But the relevant distance is the distance to the center of the planet or moon, not the distance to its surface.

The moon is smaller than the earth. So, when you're on the surface of the moon you're relatively close to the moon's center; when you're on the surface of the earth, you're relatively far from the earth's center.

When comparing the surface gravity of the earth to the surface gravity of the moon, you need to take into account this difference in distance, in addition to the difference in mass.

 

[Brown]

It's perhaps worth pointing out that the physics underlying the above discussions is pretty basic stuff. This sort of thing is often discussed in high school Physics classes. This sort of analysis has been around for about--what?--three hundred years now or so. It's been tested again and again and again.

If the fundamentalist's point is that "science can't explain everything," then this example is one of the most foolish that s/he could have selected.

 

[pupdog]

Perhaps they have an alternative theory they wish to intrude into science classes.

 

[zeno]

Darwinism was no more the end of something then the Model T Ford or light bulb. It was the beginning of something. Science, endlessly evolving , always has more questions then answers.

Fundamentalists, devolving, however, rule the world . It might be a good idea to keep that in mind.

 

[Kimpatsu]

If the fundamentalist's point is that "science can't explain everything," then this example is one of the most foolish that s/he could have selected.

True, but then again, since when have fundies been known for their intelligence?

 

[Zep]

The hollow earth, people! Don't forget the hollow earth! Oh, and "Becuz Gawd Sed" is considered "quite scientific" in some circles too.

 

[Kimpatsu]

The hollow earth, people! Don't forget the hollow earth! Oh, and "Becuz Gawd Sed" is considered "quite scientific" in some circles too.

When I was in high school, a group of us found the hollow earth idea so risible, we started our own chapter of the Hollow Earth society, and wrote daft, windup letters to the real afficionados. For example, we wrote to Brinsley le Poer Trench and addressed the letter, "Dear Uncle Brinsley", and there was a guy from America who wrote to us and mentioned that he was "semi-retired", so we wrote back calling him "semi-retarded".
We even published our own newsletter for over a year, which was a mixture of silly jokes about the teachers and demands for The Outer Limits and the Twilight Zone to be shown continuously on late-night television. At age 16, this was the funniest thing on earth (hollow or not...)

 

[daenku32]

Ah. That explains what I read a week ago about black holes and their effects on things. How a smaller black hole would actually have different effect on the matter around the event horizon than a bigger black hole.

edited to add:
So I guess standing on the surface of an object of 1 earth masses but a radius of only 1 mile would cause you to flatten immediately.

 

[Hand Bent Spoon]

That guy on the 700 Club or some such fundy TV show who wears a lab coat and tries to act all scientisty (disclaimer: not a real word) bugs the hell out of me.

And the fundie's train of thought is, if science is wrong about anything, then the Bible is right about everything. The thing I hate most about religion is the anti-intellectualism of it.

 

[ebola]

It really is a shame that Jedi Knight has been banned; here is yet another opportunity for him to display his brilliance by demonstrating his expertise on this subject matter as he did so hilariously on Franko's "Gravity" thread (since pruned). Anybody out there got copies?

Eric