# History of gravitational law

1. Feb 19, 2015

### Avichal

I'm trying to understand on how people came up with gravitational law i.e. observations, experiments, theories etc. I have read a bit but there are some blanks that I cannot fill. I need some help regarding that.

It seems that people like Hooke, Newton already knew that there exists gravitational force between every two objects. And that it is proportional to their masses and inversely proportional to square of distance between them. This was confirmed by Newton based on Kepler's laws.

But I want to know how did they know this before Kepler's laws? What experiments did they do to come up with this? How did they come up with the concept of mass in the first place?

2. Feb 19, 2015

### Staff: Mentor

They didn't know there was a force between two objects prior to Newton. They just knew that objects fell to the ground at a particular acceleration. They also had no idea what kept the planets in their orbits.

3. Feb 20, 2015

### sk1105

The concept of gravitational mass came about because some objects exert a stronger gravitational force than others, so they must have some attribute that governs this i.e. the quantity we call mass. This is different from the concept of inertial mass, which describes how much work is required to change an object's motion. In reality, though, these two masses are equivalent, which is why all objects fall with the same acceleration (neglecting air resistance).

Kepler came up with his laws of planetary motion based on years of observations of the planets made by Danish astronomer Tycho Brahe. Later on, Newton invented a new form of mathematics (calculus) which he used to write his famous laws of motion, as well as the law of gravitation. He found that Kepler's laws were a consequence of his more fundamental laws.

The well-known tale about Newton and the apple is likely apocryphal, but he did have a moment of inspiration when he reasoned that the force pulling an object down towards Earth is the same force that stops the Moon escaping into space. It was this shift in perspective, from the division between an imperfect Earth run by humans and an unchanging heavenly realm run by God to a unified system governed by the same laws, that ultimately led Newton to his law of gravitation.