# Physics: What is Gravity?

What is gravity? On a day to day basis, most people do not even think about it. It is one of the most powerful forces in the universe. Gravity affects the universe as a whole, it keeps our planets in orbit, and it affects everything that happens in our universe.

## Gravity Defined

In its most basic definition, gravity is the natural force through which a physical body attracts another object towards itself. Three items determine the force of gravity— the masses of the two objects and the distance that separates them. The force of gravity acting on objects gives them weight. Weight is simply a way of measuring the force of gravity on an object.

## Gravitational Theory: A Brief History

Scientists in ancient times observed the effects of gravity. Most famously, Aristotle theorized (incorrectly) that heavier objects accelerate faster than lighter objects. This idea was finally disproven through the work of Galileo Galilei, one of the most famous physicists and mathematicians of the Scientific Revolution. In the 17^{th} century, Galileo conducted his famous experiments dropping balls of different weights from the Leaning Tower of Pisa in Italy. Galileo maintained that the objects, though their masses differed, would fall at the same rate of speed as long as the resistance of the air they fell through remained the same. In physics, the phrase “in a vacuum” is often used to describe situations in which an action takes place absent the effect of air resistance.

While seemingly simple, Galileo’s experiments paved the way for the work of Sir Isaac Newton, another famous physicist of the Scientific Revolution. Through his work, Newton developed what is now called “Newton’s law of universal gravitation.” Newton’s law basically states that each massive object (an object that has mass) attracts another massive object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between the two objects. Newton’s law lays the foundation for the study of physics as we know it.

From Newton’s law of universal gravitation evolved general relativity theory. The theory was first published by one of the most famous modern physicists, Albert Einstein in 1916. Einstein’s theory still holds as the current description of gravity in physics. General relativity theory describes gravity as a geometric property of space and time, more specifically, gravity as the curvature of space and time.

## Gravity of Earth

As mentioned above, gravity is responsible for keeping planets, including Earth, in orbit. Each planet has its own gravitational field, which explains why your weight would differ if you were on Jupiter rather than Earth. Earth’s gravity is basically the acceleration the Earth imparts on objects on or near its surface. In scientific measurement, Earth’s gravity, in a vacuum (absent air resistance), has a value of 9.81 meters/second/second. Gravity is often denoted as *g* in scientific equations, and can be described as *g = *9.81 m/s/s. While Earth’s standard gravitation is 9.81 m/s/s, the number can vary depending on the object’s location on Earth. The object’s latitude, altitude, and depth can all affect gravitation.

Gravity is one of the most important forces that acts on and in our universe. It is one of the four fundamental interactions of nature, and we can feel its affects on everything we do.

## Comments

I just wonder would we fly if the gravity was smaller on Earth? Interesting hub!

Nice description of gravity and the scientists that developed theories based on it.

If gravity is what causes the Moon to be tied to the earth, then what force is responsible for the velocity of the Moon in revolving around the Earth, and the Earth revolving around the Sun?