Why Gravity Gets So Strong Near Dense Objects

[EdenmanX]

Youre the best! Thank you

[MichaelShi8]

This doesn't explain why gravity get stronger, just the math behind it. No one know for sure why it works.

[MrModestmol]

Great Video! Super helpful and well thought out! I was wondering though why don't you need the radius of M2 to find Force? It seems that you are only looking for the force of M1, and the Total Gravitational Force would be greater.

[RAGHAVCOOLSTUFFS]

solved such confusin ques and explained so easily..... my mind is clear now on this topic..thanks

[fargomemorioso]

Sorry for the off-topic question, but I just want to share like you, so I want to know which program are you using. Thanks in advance, and keep going with the great work!

[09890897]

thank you so much :)

[Shalek]

The part about the center of mass, I had never thought about that. My mind has been blown.

[ZvendZved]

Big ups to Khan for all his work!

But isn't this quite obvious?

[rotateaxis]

So saturn is the least dense planet, then it has less gravity than earth?

[ramin93]

@rotateaxis Saturn is 95 x more massive than the earth, Whereas the volume of Saturn is 764 x the volume of earth. This huge volume makes the density of Saturn very low. So the gravity force on an object from Saturn is stronger than the force on an object from the earth, with respect to distance.

[Devansh22youtbe]

can you please do general relativity? I am stumped by it.

[INMATE2468]

Have u done some vids on time dialation??? Or is that too advanced? ??

[nacho1007]

thank you for sharing this

[TheBeardedThinker]

awesome

[Fupper16]

One person who disliked this passed the event horizon of a black hole ...

[xXSparky117Xx]

Thanks for this. You should do some videos on gravitational time dilation. Maybe help prove in a mathematical manner how gravity slows down time.

[guaranic]

I'm curious, is dark matter affected by gravity and does it take up space?

[Vire70]

@guaranic

I think dark matter was derived purely because the gravity in the universe doesn't make sense without it. So it's safe to say yes, it does.

[guaranic]

@Vire70 If dark matter causes at least some gravity, why doesn't it all clump in one spot?

[rolingpingu]

WOW!!!! And I though that at the center of earth, the gravity was infinite (bcuz of r -> 0), so if u had a hollow room inside, u would float at it's center (bcuz the gravity would pull u there). What I don't still get is why does the universal law of gravitation works, I mean, why do masses attract? U know, the g = 9.81, where does it come from? Since g is an acceleration, it means it comes from a force, but why do masses have an attraction force?

[ShaheemA13]

@rolingpingu thats what theoreticial physicists are trying to figure out themselves right now. one theory is that there are virtual particles called "force-carrying particles". so in this case, these virtual particles, in the case of gravity it's called the graviton, will literally carry the force(defined as a push or a pull, so for gravity: pull) from one mass to the next. the force carrying particle for the strong nuclear force is the gluon, and there are other force carrying particles as well

[Mal1234567]

Sal isn´t really answering why, he is just showing how the math works.

[ShaheemA13]

@Mal1234567 the math is based on observations made my newton. newton didn't just make that equation up. he made real life observations and found the inverse square proportionality between the force of gravity and the radius between 2 masses. "throw common sense into the wind"? common sense comes from experience. experience involves observation of the world around you. newton observed the world around him. that is where the sense in the equation comes from.

[Mal1234567]

@ShaheemA13 I'm not saying Newton didn't explain why. I'm saying that this video uses the word WHY in the title but then it only goes on to explain how the math works but not WHY it works.

[ShaheemA13]

@Mal1234567 that is WHY it works. because there is a inverse square proportionality between the force of gravity and the radius between the 2 masses that exists in this universe of ours. that's the reason WHY.

[Crozz22]

@ShaheemA13 Why is there an inverse square proportionality? Why is the formula the way that it is? Is it derived from observations only or is there an actual reason?

[ShaheemA13]

@Crozz22 it is derived from observations only. that is the reason why because we observed it to be that way. any other reason why would be a philosophical question and irrelevant to this video

[Mal1234567]

Sal demonstrates here that problems like this can't really be comprehended without a lot of mathematics. The math tells us that with r^2 in the denominator the gravity is not 1,000 times stronger in a body 1,000 times denser, it is 1,000^2 times stronger. But it doesn't tell us why this is the case. The best solution with these problems is just to go with and accept the math, and throw common-sense to the wind.

[Mal1234567]

The mass of the object located close to the center of the Earth will be *mitigated* by the mass pulling it in an outward direction. So it is being pulled inward and outward at the same time. It will therefore weigh less, but that weight concerns the NET gravitational force. At the center, the object is weightless because the NET gravitational force is zero. But it is still being pulled on from the outside.

[ChatsworthATVRentals]

Could you please explain why gravity exists?

Why is a smaller mass attracted to a greater mass.

I understand the how and why magnetization exists but I don't understand the how and why gravity exists.

[Mal1234567]

@ChatsworthATVRentals 1. gravity exists wherever there is mass, 2. the attraction works both ways, the Earth is also attracted to you very slightly. But you're not thinking in terms of geodesics which is more important, 3. nobody else knows how and why gravity exists either.

[toyreview]

@Mal1234567 Thanks for your answer to Gravity. But I cannot accept this and move on. I will continue my research. As with science and math, there is always a reason. Until we can figure this out, the creationists win, and that is not acceptable.

[Mal1234567]

@toyreview The creationists win - what? I should hope that science has a greater purpose than just the defeat of mindless superstition.

The vast majority of people will never be scientists or even scientifically mindful. Technology that comes from scientific theory has helped to keep the rabble relatively quiescent for the first time in human history. But we must remember that it was religion that got us here. I understand that prehistorical religious practice brought about numbers.

[dalems]

@Mal1234567 Absolutely not true. Religion is the result of fear and ignorance of the natural world. Every single fact that mankind has discovered is the result of curiosity and intelligence. Not one thing has ever been just "given" to us by some deity. Fearful people do not sit around and play with numbers; curious ones do.

[Mal1234567]

@dalems Which part of what I said was absolutely not true? All of it? According to Mario Livio in a book entitled "The Golden Ratio," numbers were thought to have developed from prehistoric religious rituals "that required the successive

appearance (in a specified order) of individuals during ceremonies." Now whose being ignorant? In order to combat religion, it is first necessary to understand it.

[toyreview]

@toyreview what did not tape this

[HideMyselfFrom421]

Is there anything you DON'T know, Sal?

[oexnorth]

You have quickly become one of my favorite Youtubers. Thank you, sir.

[Mal1234567]

Sal, thank you for the mathematical explanation of a situation that is completely non-intuitive!

[boeing747200lr]

Mindblowing explanation

[sarahlajolietigresse]

I saw the title for this video and thought "that sounds like a setup for a 'your mom' joke".

[Voshronogi]

@sarahlajolietigresse Kind of relevant:

xkcd.com/89/

[Sconz32]

So I can understand how the math works but it would be cool to know what is actually happening physically.

[kenng329]

how about light, it doesnt have mass, it's a wave, rite? So how do they attract each other? And where does the light go when it goes into blackhole?

[Mal1234567]

@kenng329 Attraction is really a pre-Einsteinian concept. It is more helpful to see space as a dimpled surface with lines (geodesics) that everything, even light, inevitably follows. That may not be the reality of gravity (nobody knows that anyway), but it is the simplest intuitive explanation or description. Light is considered to be a particle-wave.

[ChatsworthATVRentals]

@Mal1234567 If light is particle based, how can it pass through a solid? If light is wave based, how can it pass through a vacuum?

[Mal1234567]

@ChatsworthATVRentals mass-less particles pass right through solids. If you think the concept of a mass-less particle is strange, then you should really look into quantum mechanics to see how strange it can really get. Simply observing a qm interaction changes the result as if consciousness has some role to play in everything.

[Mal1234567]

@ChatsworthATVRentals If light is wave based, how can it pass through a vacuum?

Light waves propogate on their own constituent particles. But you're trying to conceive of light in terms of matter and waves from experience. The reality of light is much stranger than anything you or I can conceive. There are no examples around you to cause the phenomenon of light to make sense to you.

[hani6844]

Thanks Mr. Sal this lesson was very interesting . I have a question? If a black hole can absorb light then does the black hole pull faster then a black hole.

[Mal1234567]

7:42 "At every point [at the center of the sphere], it [gravity] will be pulling at you outward."

[UndeadTheta]

Sal, you should do a video on the shell theorems, they would clarify what you were saying in the end for a lot of people :)

[archaedemos]

i wonder whether i would weigh more in a "tube" through the Earth, or less , since i'm at the center. i'm thinking much less right now.

[Kastralis]

@archaedemos You'd weight less but your mass would be the same.

Weight is the effect of gravity

Mass is the amount of matter

[archaedemos]

@Kastralis thank you Kastralis :}

[Tommybotham]

And also, would we experience 0 force in the centre of a Celestial object like a star, since everything attracts at equi-radius so we feel a combined attraction of 0?

[Tommybotham]

Does the black hole move towards it's quarry whilst attracting?

[Shoyrou]

As the black hole would also be under influence of huge gravitacional forces, would it be fairly mobile? Or would its mass stop it from moving because of inertia?

[totallyfreeenergy]

Newtonian Gravity Law (being empirical)does not say much about the nature of gravitational force. all mass have an attractive force which understandably gets diluted with distance. But the fundamental question is why it is the nature of atoms/molecutes to attract (gravity)

[Melthornal]

Sal, isn't it true that you wouldn't feel any gravity once you go inside a large hallow sphere? The sphere surrounding you would cancel itself out.

[metabog]

@Melthornal As long as you were exactly in the center I guess.

[Melthornal]

@metabog No, you just gotta be inside. If you have a huge hallow sphere and you go inside, you will feel zero gravity. Say you took earth and, withough changing its mass, hallowed it out so it is only, say, 2km thick. Go inside that shell and you will feel no gravity, assuming the sphere is reasonably uniform. Closer you get to one section, more gravity you feel from the rest. It all cancels. Try doing out the math. Its pretty strange.

[Melthornal]

@metabog PS. When you do the math, think of it like a cone. You are inside the sphere, right, and the gravity that opposes itself is like a cone pointing out from you. If you get closer to one end, you less of the sphere is pulling you, but it pulls harder. The opposing surface area gets larger, but weaker. It manages to cancel out perfectly and you feel nothing.

[Shoyrou]

@Melthornal Ah, I see. Thank you for your explanation. I've asked that question for a whole lot of time xD

[Mal1234567]

@Melthornal The math you're talking about does not predict what you will feel. The math says nothing about feeling. But if there is gravity all around you, don't you think you would feel something pulling from all around you? That depends on the amount of matter surrounding you.

[Melthornal]

@Mal1234567 You feel the net force. The net gravitational force is 0. If there is some other force, and its net force isn't zero, then you would feel that. In my example of a giant spherical shell, there wouldn't be another force. Its just a giant shell full of nothing except a relatively small object, maybe a rock or something. Only force is gravity, and its net force is 0. Nothing would happen. If its full of air or something, then you will feel pressure from the fluid.  But no gravity.

[Mal1234567]

@Melthornal From this video it sounds like you'd feel the forces of gravity tugging at you from all directions at the center. Assuming you could survive the massive pressures of this sphere.

[Melthornal]

@Mal1234567 Again, I suggest you do the math out on paper. I think Sal just wasn't in the right frame of mind to think about this type of physics when he was making the video. I believe he misspoke slightly, and technically he isn't even wrong either way. As you enter a sphere you only feel gravity from the portion of the sphere closer to the center. If there is no matter closer to the center, like a giant bubble, then you are basically in free fall. No gravity.

[Mal1234567]

@Melthornal Newton treated gravitational attraction of spherical objects AS IF the pull were originating at a point in the exact center of the sphere. But one must not confuse this center point with the origin of the gravity. Imagine taking an elevator to the center of the Earth, you will find the Newtonian center of gravity will shift. When you reach the center of the sphere, the center of gravity will be all around you, no longer a point but a circle within the sphere.

[Melthornal]

@Mal1234567 Did you look at Guass's law? Gauss's law with the electrical field replaced by a gravitational field is a wonderful, simple, and pretty elegant way of looking at this situation. If you are within a hallow sphere, the gravity of the sphere cancels out all around you. As you travel deeper into a solid sphere, you can treat all the matter you have gone through as if it were a spherical shell around you, thus no gravity from it. Only the matter between you and the center gravitation.

[Mal1234567]

@Melthornal I'm agreeing with you in that the gravity of the sphere cancels out all around you. But that does not imply an absence of gravitational force on that body in the sphere.

[Melthornal]

@Mal1234567 I finally remembered the name of the theorem that proves this all :D It is the Shell Theorem. It has a pretty okay wikipedia page, although kinda hard to read. It is one of Newton's theorems, you can find a lot of different explanations all over the interwebs. It is easiest to understand with Gauss's law, I think. But if you don't know Gauss's law (and I don't know if Sal has made a video yet) then there are many other explanations as well.

[norwayte]

!Good one!

[FHomeBrew]

In the center of the earth we would feel weightless (given that the mass around us would be uniformly distributed).

[zbe8]

Nice vid. Maybe you should also point out that if instead of, for example, our Sun was a black hole with same mass than Sun, Earth would still "feel" the same gravitational force as it does now so we would orbit back hole the same way.

[Tome4kkkk]

You, sir, are a genius :) I'm thinking of downloading tens of your videos on my cell phone so that whenever I need to skip some time (waste it) in a buss or something, I'd play one of your vids to turn wasting time into time that works for me :) I only wish you could isolate e.g. highschool math from higher levels math...

[KungFuMil]

very good as usual :)

what if you have a doughnut shaped planet/star and put yourself in the middle of it? :)

[cmxcmx]

@KungFuMil if you're in the center of a donut or hollow sphere, you also get not gravitational pull, since there the mass is also distributed evenly around you.

[KungFuMil]

@cmxcmx or would gravitational pull be equal in all directions, hence you just remain floating in the centre

[clerlic]

@KungFuMil Unless the doughnut is turning inside out.

[cmxcmx]

@KungFuMil yes, but that amounts to exactly the same thing, since forces can be added together which makes opposing forces of equal strength cancel each other.

[canerdc]

@KungFuMil Universe would give a blue screen!