The Centre of the Earth and Gravity
The next question I’m going to answer is about the composition of the Earth and the fundamental force we all know an love … gravity.
So the question comes from Deb, a fellow b5 blogger at the Tangled Thread:
My son’s class is studying about various forces right now. His question at dinner last night was a two-parter: As you get closer to the center of the Earth, does it get hotter? and Does gravity pull harder the further underground you go?
First part … does it get hotter as you get closer to the centre of the Earth? You bet! The Earth’s core is over 5000 deg centigrade or about 9700 deg Fahrenheit! Man that’s hot!
The Earth is made up of the crust (that’s what we’re on) the mantle (molten and where the magma is that comes out as lava), the outer core, and inner core. As you get deeper hot only does it get hotter, but the pressure increases as well.
Wikipedia has a great article on the structure of the Earth … a good read.
Now gravity increasing as one gets closer to the centre of the Earth … thus far I haven’t been able to find a specific answer to the question, but looking up gravity on Wikipedia, leads me to believe that my initial thinking was correct … no it’s a constant.
But … that certainly was a very thought provoking question for me! We know that gravity pulls us all toward the centre of the Earth (and we actually push back, but our individual gravitational force is tremendously weak … which is why the Einstein quote "Gravity cannot be blamed for people falling in love" is so funny). So wondering if the force increases as you get closer is a bang on great follow up!
Keep those questions coming! Leave a comment or send them directly to tris AT b5media DOT com!
Tags: gravity, geology, earth structure, crust, mantle, core
April 13th, 2006 at 11:07 am
According to Newton, the gravitational force exerted between two objects is inversely proportional to the distance between them. If we use the center of the earth as the point from which we measure the distance - which seems reasonable since that is also the center of mass - then wouldn’t the gravitational force increase as you got closer to the center of the earth?
I’m gonna love this site!
April 13th, 2006 at 11:18 am
Yep you have a good point, I guess the question would be … would you notice (assuming you havn’t melted)?
April 13th, 2006 at 11:32 am
Re: gravity
I think that the gravity wouldn’t change the closer you got to the center of the earth, because the gravity that is keeping us on earth instead of floating away is coming from the Earth. It’s not like the core of the Earth is seperate from the crust, it’s all one big ball. What’s pulling us down is the gravity from the big ball as a whole, not from the middle of the ball….
April 13th, 2006 at 11:54 am
I do have an e-mail into my geology advisor from my undergrad days … we’ll see what he says!
April 13th, 2006 at 12:42 pm
If you were to travel to the center of the Earth then the gravitational forces from all the planet stuff around you would counteract each other. That is, assuming the planet is a perfect sphere (and uniform density gradient, etc).
Another fun question is what would happen if you dig a tunnel from one side of the planet all the way to the other side and then jump into the hole. Assuming there is no friction to slow you down you would start falling and when you reach the other side you would start falling again in the other direction, oscillating forever.
April 13th, 2006 at 12:45 pm
That would be fun! Okay rather toasty, but fun. ;-).
April 13th, 2006 at 12:50 pm
Better do it thru the poles, so the earth doesn’t rotate right out from under you! :)
April 13th, 2006 at 1:03 pm
But would it rotate away from you … might the gravitational pull be enough to keep you from achieving escape velocity? So would you keep yo-yoing through the mantle (ouch that would suck)?
April 18th, 2006 at 9:37 am
No way, you will always have zero velocity at the surface and maximum velocity at the center, so it won’t be risky in a sense of falling from the Earth.
April 18th, 2006 at 10:02 am
Boy I can’t wait for the kiddo to come home from school so I can read him the answer and all the comments too! *g*
April 18th, 2006 at 10:22 am
I figure gravity will be strong enough to keep the earth from speeding away as it revolves around the sun; but what about rotation on its axis? Won’t you stop rotating with the earth? What force would pull you along if you aren’t in physical contact with the surface? I suspect you might even bump into the sides of your hole.
April 18th, 2006 at 12:13 pm
Nope … since gravity is a force between bodies, no need for contact. The moon’s gravity causes the tides on Earth and the Earth’s gravity still affect planes in the sky.
Gravity isn’t like electricity where contact is optimal, it just is. Oooh how Zen is that!
April 18th, 2006 at 12:49 pm
I know contact isn’t relevant for gravity (except to the extent it is measures distance between the two bodies), but is it for friction. It’s like standing on a stationary object that suddenly starts moving. Depending on the force of friction between me and the now moving object, I will either stick with the object or the object will slide out from under me.
Now, let’s assume I’m standing on an object that’s already moving. If I jump, air friction will slow me down, and the object will move out from under me.
Next step: Make this already moving object the earth. As I fall back and forth thru our imaginary hole, my lateral movement will eventually become different than the lateral movement of the hole on the earth surface. Air friction will slow me down, and my hole will rotate out from under me. Won’t it?
Follow up question #1: Why don’t we feel a breeze as the earth rotates?
Follow up question #2: What keeps the earth rotating? Why doesn’t it slow down?
April 18th, 2006 at 10:45 pm
#1: The earth rotational velocity at the equator is 465 meter/second (or 1 674 kilometer/hour). How do you think you don’t notice such a speed? Because the earth’s atmosphere rotates with the earth as well, with the same velocity. So, if you jump, atmosphere won’t slow you down, as it moves with the same velocity as you. However, the rotation of the earth has some effects on the atmosphere. Did you know that cyclones rotate counterclockwise in the northern hemisphere, and clockwise in the southern hemisphere? This is due to Coriolis effect, which appears in any rotating object.
#2: Why should the earth slow down if there is an empty space all around it? There is no friction between the earth and outside vacuum, so no slowing down.
May 6th, 2006 at 8:01 am
If I was to stand in the very center of the Earth, which way would gravity pull me? Or would I be just floating not being able to go any which way?
May 6th, 2006 at 8:25 am
Floating, no, but you would be stuck. Since gravity pulls you towards the centre of the earth, that’s where’d you be!
October 2nd, 2006 at 6:55 am
Hello,
Questions: as we exit the atmoshpere of the Earth, do we not experience less gravitational pull, even zero gravity? Or is it that we come closer to other celestial bodies that then attract us?
Is there a black hole in the center of our galaxy that holds it together? Is the force of gravity equal thoughout the galaxy? Or stronger in the center of the galaxy? I know we are on the far edge of ours.
January 11th, 2007 at 10:04 am
[…] I decided to pick a few of my favourite posts (it was hard, because I like a lot of them) to highlight. The first up is the post that still holds the record for the most comments on any one post, a discussion of gravity and the centre of the Earth. Never would have thought that a discussion of traveling through the centre of the Earth would generate such an interesting comment flow. […]