# What is Gravity

Gravity is the force that holds our solar system together. Without it, planets would simply float off into space, remaining un-deviated until they collide with another body. The detailed study of gravity began with Galileo, who was the first to notice that all objects, regardless of their mass, are accelerated at the same rate. Isaac Newton published a theory of universal gravitation which correctly describes the motion of the planets, the moon and the tides. Newton’s methods are sufficiently accurate, and will be used in this article. However, his work has been superseded by Einstein’s general relativity which is even more accurate, but more difficult to use.

A commonly held misconception is that only very large bodies, such as planets and stars, exert gravity. In reality all objects, regardless of their size, exert gravity. However, for most everyday objects, that gravity is extremely small and can be deemed insignificant. The gravitational field strength of an object is proportional to its mass, i.e. the larger the object, the larger the gravitational force it exerts. Large ships, such as aircraft carriers, have enough mass to exert a significant amount of force. If two ships, each weighing 35,000 tonnes, separated by 100 metres of water, will exert a force on each other of over 8.2 Newton’s. This force, though relatively small, shows that even a small (when compared to planets) object can exert a significant amount of gravity.

The gravity exerted by object falls in proportion to an “inverse square” law when the distance from the object increases, i.e. if you move twice as far away, the force quarters. This means that a force produced by gravity falls rapidly at a distance. One consequence of this is the gravitational field strength at the north and south poles being higher than the field strength at the equator. The reason for this is that the earth is not a perfects sphere. The poles are closer to the centre of the earth than the equator; hence the force of gravity at the equator is less.

Although the force of gravity falls rapidly at a distance, the sun is large enough to have a significant effect on the earth. It is the sun’s gravity which keeps the earth (and the other planets) in orbit around it. The sun’s gravity is also strong enough to have an effect on the tides. The sun causes two tides per day, while the moon cause two tides per 25 hours. If these tides are in phase (together) then a spring tide is produced. If these tides are out of phase then neap tides are produced.

If an object has a gravitational field, then there must be a velocity which another object must exceed to escape the pull of that field. This is called the escape velocity. The escape velocity is defined as the minimum velocity that an object must have to escape the pull of a gravitational field. For most objects, the escape velocity is negligibly small. For planets, the escape velocity is quite high. The escape velocity depends on the mass of the object from which you are trying to escape, and the distance from the centre of that object. It is important to realise that this velocity is for objects which do not have extra propulsion, for example if a cannon fires a ball directly upwards, the escape velocity is the velocity that the ball must have to escape the earth’s gravity and get into space. If an object like a rocket is travelling upwards, then the escape velocity is not relevant to it, since it has a continuing source of propulsion. There is still a minimum amount of propulsion it requires to leave the earth’s gravity, but this is completely different from the escape velocity

If an object has sufficiently large mass, or sufficiently small radius, its gravity can become so strong that it’s escape velocity is faster than the speed of light. In this case, nothing can escape from the object, and it becomes a black hole. It is called a black hole because light cannot escape from it, hence it appears black. It is worth noting that as light is emitted from the surface, its velocity does not decrease as it heads upwards. Instead, the energy of that photon of light decreases, and this decrease in energy will be observed as a change in wavelength. The physics of black holes cannot be explained using Newton’s theory, and is instead described by Einstein’s theory of general relativity.

The study of gravity covers the entire universe. Gravity plays a vital role in the way the universe is formed. It has a huge effect on our lives, and without it, arguably, we would not exist. The study of gravity has evolved greatly in recent times, from Newton’s theory of gravity which is accurate and simple to calculate, to Einstein’s general relativity, which is even more accurate (and more difficult to use), and describes the things that Newton’s theory cant. Gravity will continue to play a vital role in the field of physics, and is sure to perplex many a scientist for years to come.