Introduction to Classical Mechanics by Stephen Hughes
Gravity, a natural phenomenon that keeps our feet firmly on the ground and occasionally permits us to have our heads in the clouds, shapes the Universe by binding vast systems of planets, stars and galaxies together. It is also responsible for the Earth’s dynamic tides and causing apples to fall from trees. Yet it is the weakest of all the fundamental forces that govern how everything interacts with everything else. The other three forces are the strong force, electromagnetism and the weak force, in order of decreasing strength. The strong force binds protons and neutrons together forming the nuclei of atoms. Electromagnetism is responsible for electric charges attracting and repelling one another. This also includes the similar effects experienced by magnets. The familiar behaviour of this force is how opposites attract and likes repel. Finally the weak force, still far stronger than gravity, causes radioactive decay. Why then does gravity have such a big impact on the dynamics of the Universe? The answer to this question has two parts. Firstly the strong and weak forces act only over very short distances, about the same distance as the size of an atoms nucleus. Secondly, although electromagnetism has an infinite range it acts on objects that have either a positive or negative charge. There are usually an equal number of positive and negative charges in a given region so any overall effect cancels out. Gravity in comparison has an infinite range but acts only on objects that have mass, which is always positive. This is how gravity triumphs in governing the large scale dynamics of the Universe. Only one type of mass, positive mass, means gravity causes everything to be attracted to everything else.
When Isaac Newton described gravity mathematically he provided a method to calculate the future position of the planets, the height of the tides and eventually helped land a spacecraft on the moon. The downside to his description is that it gives no indication of how gravity works. What mechanism causes two objects with mass to be attracted towards each other? As astronomical measuring instruments improved it was observed that the predictions regarding Mercury’s position were slightly wrong. Albert Einstein had been thinking about gravity and was interested in this problem with Mercury. His new theory of gravity, general relativity, accounted for this small difference and calculated the correct position of Mercury. It does this by incorporating the strong gravitational effects experienced near to very heavy objects, like the Sun. Mercury, being the closest planet to the Sun, is affected more than any other planet in the solar system. General relativity also gives us an insight into the mechanism that makes gravity work. Space is described as distortable, compressing and stretching in the presence of mass. Space warps around an object with mass, which causes a passing object to follow the shape created by the warped space, giving an illusion of a force acting between the two objects.
Stephen will be leading a fascinating course by Continuing Education titled an Introduction to Classical Mechanics: The Origins of Science from Wednesday 1 October – http://payments.liv.ac.uk/browse/extra_info.asp?compid=1&modid=2&catid=33&prodid=509