**Explainer: What is gravity?**

+ We barely think about it, but gravity defines how we interact with our world. We grow up within its constraints, and our muscles, balance system, heart and blood vessels all depend on it. Gravity literally grounds us — but what exactly is it?

**+ The basic idea**

+ Physicists see gravity as one of the four fundamental forces that govern the universe, alongside electromagnetism and the strong and weak nuclear forces. A force is defined as an interaction that changes an object’s motion, and so these four forces underpin all of physics and define how everything in the universe interacts – from the vast cosmic interplay of galaxies to the tight bonds that bind quarks inside a proton or neutron.

+ Gravity is the weakest of these forces, but it’s the one we’ve been aware of for longest. For centuries, we knew that our feet are kept on the ground and the planets are kept in orbit around the Sun. Even before gravity was described mathematically, seventeenth-century astronomer and mathematician Johannes Kepler had formulated accurate laws to predict the motions of the planets.

+ Unfortunately, no one had any clue why the planets are orbiting in the first place.

**+ Newton’s law of universal gravitation**

+ Enter Isaac Newton, who realised there must be a force acting between the planets and the Sun. (He also defined what a force is.) Whether or not a falling apple really prompted his eureka moment, the equation he came up with to describe the behaviour of this force was revolutionary.

*F* = *Gm*_{1}*m*_{2 }/ *r*^{2}

+ This equation says that gravity is a force that two objects with mass exert on each other simply because they have mass. The strength of the force (*F*) is proportional to the masses of the two objects (*m*_{1} and *m*_{2}) divided by the square of the distance between them (r). The *G* is a constant that measures the basic strength of the force.

+ It boils down to this: the more massive objects are, the greater the force of attraction between them, but the further they are apart, the weaker the attraction.

+ Consider the legend associated with Newton’s revelation about gravity: an apple falling from a tree. Newton’s law of universal gravitation tells us that not only does the Earth tug on the apple, the apple also tugs on the Earth. But the Earth’s enormous mass means it takes a lot more force to move it an appreciable amount, so the apple comes toppling down while the Earth remains practically motionless. The same is true in a broader context. Every object in the Universe is attracting every other object, but the closer and more massive it is, the greater its gravitational power…

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