gravity is a **attractive force that appears between two bodies with mass**, stronger the greater the amount of mass. It is a natural phenomenon considered as one of the four fundamental interactions of matter together with electromagnetism and the strong and weak nuclear interaction forces.

Gravity, also called gravitation or gravitational interaction, is not a very strong interaction. In fact, **is the weakest fundamental interaction of all** (10^{36} weaker than the strong nuclear interaction and 10^{29} weaker than the weak nuclear interaction). Very massive objects such as planets, stars and galaxies are required to be experienced by the human senses, but it is practically irrelevant at the subatomic level.

The force of gravity exerted by an object depends above all, as mentioned, on its mass, but also on distance: the further away two objects are, the less they are affected by each other's gravity.

According to Einstein's explanation for the phenomenon of gravity, the attractive force can be understood as a deformation of the space-time fabric that produces the presence of the object's mass. This deformation creates a gravitational field around the object whose intensity is **proportional to mass** of the object p**ero decreases inversely proportional to the distance**.

A small object with little mass that enters the gravitational field of an object of comparatively much greater mass will suffer a **acceleration** which brings you closer to the massive object faster and faster. This acceleration near the surface of planet Earth is **approximately equal to 9.8 m/s ^{two}**which means that an object in free fall would experience a speed increase of 9.8 m/s every second towards the center of the planet.

## Intensity of the earth's gravitational field

The strength of a gravitational field is represented by the letter **g** and is measured, in the International System, in **N/kg (newtons per kilogram)**, units that express the attractive force exerted by an object per kilogram of mass. It is also expressed equivalently as acceleration in m/s^{two}.

On a spherical planet, the intensity of the gravitational field on its surface can be calculated with the following formula:

Where:

*g*is surface gravity._{His p}*G*is the universal gravitational constant (6.674×10^{-eleven}N m^{two}/kg^{two}).*R*is the radius of the planet.*or*is a unit vector (value 1) with direction to the center of the planet._{r}

It can also be obtained by the ratio of the weight of an object on the planetary surface between its mass:

The average strength of the earth's gravitational field at the planet's surface, known as **standard gravity** It is **1 N/kg** (sometimes represented as **1g**), **equivalent to an acceleration of 9.80665 m/s**** ^{two}**.

### Variations in Earth's Gravity

The intensity of the earth's gravitational field is not homogeneous throughout its surface. This is due to several factors, including:

**Latitude**: Gravity is affected by the planet's rotation on itself and the planet's not perfectly spherical shape (flattened at the poles).**altitude and depth**: the earth's surface is irregular presenting areas farther from the center of the earth (mountains) and closer areas.**Density**: the density of the planet is not homogeneous.

Latitude is one of the most important factors in changes in the intensity of the gravitational field at the earth's surface. Due to the rotation of the planet on itself, the objects undergo a **outward centrifugal force**, contrary to gravity whose direction is towards the center of the Earth. The centrifugal force is strongest at the equator and decreases as we approach the poles where the axis of rotation is located. Consequently, **gravity at the equator is less** being counteracted by the greater centrifugal force.

The lower intensity of the gravitational field at the equator is also due to the fact that gravity decreases with distance and the planet is not a perfect sphere, it is flattened at the poles, so the objects at the poles are closer to the center of the Earth and further away if they are located on the equator.

Adding the effect of the rotation and the shape of the planet there can be a **5% difference** between gravity at the poles (approximately 9,832 m/s²) and gravity at the equator (9,789 m/s²).

Gravity also decreases with altitude and increases with depth. In this sense, the topography and geology of the area influences the measurement of local gravity. For example, the presence of mountains or a composition with **rocks of greater or lesser density** are factors responsible for changes in gravitational intensity between areas located at the same latitude.

At the surface of the Earth's core, the intensity of the gravitational field increases to 10.7 m/s^{two}. If we were located right at the center of the earth, gravity would be null, since the intensity of the gravitational field is a vector magnitude and at the center of the sphere all vectors would cancel.