# Forces: Gravity

## Gravity - What, How and Why?

In the Lander game, gravity is the downwards force on your lander spacecraft, it changes between each level. This page explains how and why this happens.

### What is gravity?

Gravity, the weakest of the four fundamental forces, causes all objects with mass in the universe to be drawn to one another. It is what makes things feel heavy, keeps planets in orbit around the sun, and stops us flying off into space when we jump up in the air. It is always an attractive force and without it stars would never have formed and you would not be here reading this.

Gravity is a mystery. No one really knows why a ball will fall to the ground when dropped from a height but there are several well received theories that attempt to explain it. The two most widely held theories of gravity come from Isaac Newton and Albert Einstein.

### Theories of gravity

Isaac Newton was the first person to develop a cohesive explanation for gravity, by defining it as predictable force that acts on all matter in the universe. His theory states that each particle of matter attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportionally to the square of the distance between them. This sounds horribly complicated, but simply means that the gravitational force will be weaker between objects that are further apart or objects that are less massive.

#### Newton's law of universal gravitation

Newton’s law is an excellent approximation for the effects of gravity in most applications, however Einstein’s theory of general relativity is required when more precise measurements are needed or for example when dealing with very strong gravitational fields.

Einstein described gravity as the result of the curvature of space-time, as opposed to a force. Space-time is a mathematical model which combines space and time into a single, four-dimensional interwoven continuum.

If you imagine setting a large body, for example a rock, in the centre of a trampoline: it would press down into the fabric, causing it to dimple. A lighter body, for example a marble, rolled around the edge would spiral inward toward the body, pulled in much the same way that the gravity of a planet pulls at rocks in space.

An example of when general relativity is needed over Newton’s law of Universal Gravitation is in the case of Mercury’s orbit. Due to Mercury’s proximity to the Sun, it orbits in an area where the space time is heavily distorted, and therefore using Newton’s approximation is not enough to fully predict the orbital motion of the planet.

Though general relativity isn’t the only relativistic theory of gravity, it’s the simplest theory that is consistent with experimental data. However, many problems remain, the most fundamental being: how can general relativity be resolved with the laws of quantum physics? So the mystery of gravity’s pull is very much still unresolved; but we can understand how it works. By using this information you should understand that a much bigger object like Jupiter will have a much larger gravitational force than the Moon.

### External links

- Minutephysics: What is Gravity? (video) - A quick and easy-to-understand introduction to what gravity is and how it works.
- Crash Course: Newtonian Gravity (video) - A more in-depth explanation of what Newtonian gravity is, how Kepler's laws explain the orbits of the planets and their moons, and also how these laws are still in use today.
- NASA: What is gravity? (webpage)
- Einstein's theory (webpage)