Why does gravitational field strength decreases below the surface of the Earth?

Question

Gravitational field strength increases above the surface of the Earth. Yet it decreases below the surface of the Earth, why is this? I know it has something do with having a force above you.

Answer 1

When you are inside the earth at some depth say r where the radius of earth R then the Effective gravity of pulling you experience @ that depth is 4 X Pi X G X density X (R-r) .density here is the earths density and G is gravi constant .Consider slicing off part of the earth that that lies above you and call the part that looks like "C" as 'A' the other half as that looks like like D as 'B'..
'A' part of the earth that lies above you to its extent exert an opposite pull i.e upwards.It gets cancelled EXACTLY by the force exerted by the hallow encircling the sphere (of R-r radius.) in 'B' (The sphere completely lies in B as you should have imagined) The only *EFFECTIVE* part that exerts the acceleration is the sphere inside B HAVING the RADIUS.(R-r) hence the equation "4 X pi X " etc..is given above.


By the way DO check the link given below and search for "sky-diver" question .The asker has asked how the sky diver will dive through endless tunnel on earth and notice the physicist's answer "he experiences less and less force as he goes deeper BECAUSE there is LESS and LESS of the earth PULLING on him ".whatever you do, DO notify me if i'm wrong!!!!!

Answer 2

1. D 2. C (don't know about how frequent "frequently" really is, though. We see auroras here in Pennsylvania a few times a decade or so) 3. B Gravitational force acts as the inverse square of the distance (twice as far, 1/4 force) 4. C 5. A is best, but remember Kepler followed Kopernik (the real, Polish name for the guy we call Copernicus) by many years.

Answer 3

The force of gravity is given by the formula F = G Mm / d^2.

The distance d is the distance between the two masses M and m.

For earth, we assume that the whole mass is concentrated at the center of the earth.

Therefore, the above formula when applied to the surface of earth is

GMm / R^2, where R is the radius of earth.

On any point on the surface of earth, this is the same.

But inside the earth this formula is not valid. It is because we assumed the mass of earth to be concentrated on the center.

When a point inside the earth is considered, it has masses both toward the center of earth and away from the center of earth.


Consider a point A in between the center of earth and a point B which is on the surface of earth. O is the center of earth.

Join B A O C. C is the other end of this line and it is on the opposite surface of earth.

Divide the earth into two portions by cutting it using a plane passing it at the point A and perpendicular to BAOC

One portion has less mass than the other. The center of mass of one portion is in side BA and the center of mass of the other portion is inside AOC and away from O.

If the test mass is now placed at A. It is pulled by two forces one due the mass whose center mass is on BA and the other due to the mass whose center of mass is in the line AOC.

The resultant force is less than that the force that was acting on the surface of earth.

As the point approaches the center of earth, the force decreases and becomes zero at the center point.

At the center, the two masses on either side are equal and their center of masses is also equidistant.

Answer 4

Gravitational field strength is directly proportional to the effective mass acting on the object at any given location. As the object moves towards the centre of the Earth, the effective mass acting on it decreases (the volume of the sphere is 4/3¶r3 and the radius to the centre of the planet is decreasing).

Answer 5

Actually the gravitational field intensity decreases as you rise above the earth and it increases as you sink beneath the surface. At the centre of the earth the gravitational field intensity is at a maximum!

Now.... gravitational FORCE (with is a different though associated concept), is proportional to the rate of change of the gravitational field intensity with respect distance. At the centre of the earth the rate of change of the gravitational field intensity with respect to distance is zero, so the force of gravity at the centre of the earth is zero!

Correspondingly the force of gravity is less below the surface and reaches a maximum at the surface! Above the surface of the earth both gravitational field intensity AND gravitational force decrease as height increases.

Hope that helps. Email me if not.

Answer 6

When you are standing on top of a mountain, you weigh a tiny fraction MORE than you do at sea level, because there is more mass between you and the center of the Earth. Gravity is a function of mass. More mass = more gravity.

The gravitational field does NOT get stronger above the surface of the Earth. If anything, it would get weaker.

The force of gravity is inversely proportional to proximity. In other words, the farther you are from the center, the less gravity you feel.

It is true that we can measure minute changes in gravity below and above the Earth's surface. As you get closer to the core (center of mass) you have less mass pulling on you. As you stand on the mountain, there is more mass pulling on you. As soon as you separate yourself from the surface and move away from the planet, as an astronaut does, you get lighter, not heavier.

As to the idea that you "know it has something to do with a force above you" well, ...you are mistaken.

There is no pressure from above. Gravity is not a pressure or repulsive force from above, Gravity is an attractive force. (Ms. Gravity looks great in high-heels and a black dress!)

All of Newton's equations work perfectly in our local universe. Those equations deal with gravity as an attractive force. Since they work so perfectly, we assume that gravity is an attractive force.

Even Einstein's equations which deal with gravity do not use the idea of pressure. The Einsteinian view of gravity is that, on large scales, gravity warps the fabric of space, much as a bowling ball would depress the surface fabric of a trampoline.

But even those revolutionary ideas and equations still deal with gravity as an attractive force in the sense that objects tend to move toward the center of mass as golf balls would move toward the heavy bowling ball in the center of a trampoline. There is no dichotomy between Einstein and Newton. Their theories simply apply to different scales. Both theories accept the premise that gravity is attractive.

All of these equations work perfectly in their respective scales. Absolutely every experiment so far verifies these equations and their predictions to be true. Therefore, until we know something new and different, we think the underlying assumptions of these theories are true.

Gravity behaves as an attractive force.

Although we have no proof yet of exactly how that force operates. When the super colliding particle accelerator at CERN starts up, that is one of the first questions on every scientist's mind, "How does gravity work?"

They will be looking for gravitons, a theoretical particle that transmits the force of gravity. Until then, we haven't a clue. However, we are very certain that gravity is attractive and that it gets weaker by the square of the distance from central mass.

We CAN make incredibly accurate super-fine measurements of gravity. Here is what we know for certain;

You get heavier if you STAND on a mountain.
You get lighter if you stand in a deep subterranean cave.

That’s because of the relative amount of mass between you and the center or core.

As soon as you leave the surface of any planet, you start to get lighter, not heavier.
When you get far enough away from the core, (above) you weight almost nothing.

One hundred percent of every scientific experiment agrees with these findings.
There is absolutely no evidence of a “force from above.”