# Physics equations/06-Uniform Circular Motion and Gravitation/A:history

#### Newton's law of universal gravitation[edit | edit source]

Newton published this in 1687, his knowledge of the numerical value of the gravitational constant was a crude estimate. For our purposes, it can be conveniently state as follows
^{[1]}:

**Solution:**

- is the force applied on object 2 due to object 1
- is the gravitational constant
- and are respectively the masses of objects 1 and 2
- is the distance between objects 1 and 2
- is the unit vector from object 1 to 2

(Note: the minus sign is a complexity that is often ignored in simple calculations. Don't fuss with minus signs unless you have to.)

Since the magnitude of the unit vector is "one" , the unit vector vanishes when we take the magnitude of both sides of the equation to get:

.

#### Weight and the acceleration of gravity[edit | edit source]

The force of gravity is called **weight**, , If one of two masses greatly exceeds the other, it is convenient to refer to the smaller mass, (e.g.stone held held by person) as the **test mass**, . A vastly more massive body (e.g. Earth or Moon) can be referred to as the **central body**, with a mass equal to . It is convenient to express the magnitude of the weight () as,

,

where is called the acceleration of gravity (or gravitational acceleration). Near Earth's surface, is nearly uniform and equal to 9.8 m/s^{2}. In general the gravitational acceleration is a vector field, meaning that it depends on location, * g = g(r)* or even location and time,

*.*

**g**=**g**(**r**,t)#### Gravity as a vector field[edit | edit source]

under construction

- define the vector field for a single massive point object
- make analogy to magnetic field as that which causes a torque on a magnet
- mention temperature and wind velocity fields in meteorology
- perhaps mention the need for vector calculus on a spherical object (problem solved by Newton, I think)

#### How G was actually measured[edit | edit source]

under construction: keep it brief and include an image and a reference to good wikipedia article