Close

Culture Online

Home

# What is the scientific proof for gravity?

What a great question!

29 March 2021

An apple falling from a tree! That’s all the proof you need – indeed everything falling to the ground tells us that gravity exists. But it not only works between objects and the Earth, it works between any two things with mass, be they galaxies, planets, apples, or even you and me. But we just can’t feel the gravitational attraction between us because it is a very weak force, so the gravitational attraction between you and me is incredibly small, while the much more massive force between you and the Earth is what dominates.  The relation for this force is called Newtons Law of Gravitation which has the equation:

Where F is the force between two objects of masses m1 and m2 separated by a distance,  G is the Gravitational Constant (6.67 x 10-11 m3 kg-1 s-2)). You can immediately see that the force will be greater the more massive the objects, and the closer they are together.

So when you drop an apple to the floor, it falls because it is being pulled by a force F as a results of the two “objects” being attracted to each other - the apple and the Earth. The apple stops because it hits a barrier, the floor, but would otherwise continue falling. Similarly, we fall to the ground when there is no barrier to break the fall.

Lets look at this: the gravitational force between to people of, say, 70 kg mass, standing a metre away from each other is, according to the formular above, 6.67 x 10-11 x 70 x 70)/(1 x 1) = 3.27 x 10-7 Newtons – ie., not much (it would take about six hours in a vacuum in space for two people a metre apart to come together); on the other hand, the gravitational attraction of a person to the centre of the Earth is (6.67 x 10-11 x 70 x 5.972 x 1024)/(6371000 x 6371000) = 687 Newtons. In fact, this approximation can also be validated by saying if the force pulling a person to Earth is 687 Newtons, the acceleration due to gravity, g, can be found from F = ma = mg; so g = F/m =687/70 = 9.81 ms-2 which is exactly the correct value. Not bad for a grossly approximate calculation!

The actual proof of gravity and the validity of the equation above came with the Cavendish experiment in 1797, when Henry Cavendish set up two large and two small lead balls, and observed the gravitational pull between them with a telescope. A similar classroom experiment can be seen here:

Astronomers see gravity in action all the time – planets around stars, stars around galaxies and even black holes – the most massive things in the universe – are observed by looking at their gravitational pull on everything around them. They are so massive with such a huge gravitational force that light itself cannot escape their gravitational clutches – so they are completely black. They really are the point of no return...

The Milky Way’s black hole, Sagittarius A* (Credit Abhishek Joshi / UIUC.)

A real observed black hole in galaxy M87 (Credit Event Horizon Telescope collaboration et al.)