CHAPTER 2 HOW SCIENCE WORKS ASSIGNMENT

MAKING A THEORY – GALILEO GALILEI

Galileo Galilei – perhaps the only scientist to be known by his first name – was born in Pisa, Italy, in 1564, the same year as William Shakespeare. His father was a nobleman, interested in philosophy and music. As a boy, Galileo wanted to be a musician, but his father sent him to university at Pisa to study for a much better paid profession: medicine. There, he happened to hear a lecture about mathematics that changed his course of study and his life.

His first job was as a poorly paid lecturer in mathematics in the university, but he was an argumentative young man (then aged 25) and became quite unpopular. This was to some extent the story of his life – he was pretty good at annoying people who happened to disagree with him. So he moved to the university at nearby Padua, where he stayed for the next 20 years.

Galileo’s work linked the physics and mathematics of motion with astronomy. First, he felt he had to sort out the errors and inconsistencies in the current theories of physics. These were based on the writings of the highly intelligent philosopher (and early scientist) Aristotle. But they were by now of course almost 2000 years old, and although remarkably convincing for their time, they were not based on experiments. Philosophical ideas, such as the perfection of certain geometrical figures, especially circles, were also linked with semi-religious ideas, such as the impossibility of a Supreme Being tolerating the existence of ‘nothing’ – a vacuum. Aristotle’s ideas are quite complicated, and many of Galileo’s fellow mathematicians did not understand them particularly well – as Galileo often pointed out to them in debates and eventually in his books and pamphlets. In those days, areas of disagreement were argued over rather than put to the test of experiment. Galileo was good at arguing, but he also made experiments to test his theories – a novel way of doing things, which weren’t as convincing to his contemporaries as they might be to us.

One of Galileo’s major contributions to the study of motion is described on page 24 of the book. He devised a system of smooth planks of wood at an angle to each other, and rolled a metal ball down one side and watched how far it rose on the other. He then altered the slope of the second plank and concluded that it would rise no higher than the height it had started from. Ideally, he thought, it should rise to exactly the same height. To us, perhaps, this seems a very simple, even trivial, idea. But the physics involved is quite complex – involving the concepts of energy and its conservation, inertia and, of course, gravity. Galileo had no clear idea of these concepts (after all, even Newton had no concept of energy). He further deduced from his experiment an idea that was truly revolutionary for his fellow mathematicians and physicists: if the second plank was made horizontal, the ideal frictionless ball would carry on moving forever! This idea was the first tentative formulation of Newton’s first law of motion, and of inertial frames of reference.

Later, Galileo used this idea to browbeat his contemporaries, who believed that, for example, the Earth couldn’t be moving around the Sun because there was nothing to push

CHAPTER 2 HOW SCIENCE WORKS ASSIGNMENT

it. ‘Everybody knew’ that, when an object, however frictionless, in practice came to a stop, it had run out of what Aristotle’s followers had called ‘impetus’. Tom and Jerry – and the Roadrunner – often illustrate this idea. They run off a cliff or a roof and carry on going horizontally until they look down – they’ve lost impetus and down they go. Galileo applied his ideas on motion to the movement of cannon balls. The old impetus idea is shown in Fig 2.A1, Galileo’s idea in Fig 2.A2. The continually war-making rulers of Galileo’s day were much impressed by this extra accuracy now obtainable in ranging guns, and Galileo found his skills much in demand.

Fig 2.A1 Newton’s third law. If one child pulls or pushes, both move

[Insert Fig 2.A2 from artwork brief]

Fig 2.A2

1. What major contribution did Galileo make to the style of scientific decision-making?

2. Think about the paths in the air of a thrown cricket ball and a badminton shuttlecock. Which corresponds toa) a Galilean model,b) an Aristotelian one?How, as a Galilean, would you explain the path of the shuttlecock to a sceptical Aristotelian?

3. Aristotle’s theories about motion had been accepted by intelligent people for almost 2000 years in Galileo’s time. One of his predictions was that a falling object (or a ball rolling down a slope) would gain a speed that was proportional to the weight of the object: ‘heavy things fall faster’. How could you adapt Galileo’s sloping plank

CHAPTER 2 HOW SCIENCE WORKS ASSIGNMENT

experiment to check whether or not this is true? What degree of accuracy would you expect in your experiment?

4. Galileo was criticised at the time for neglecting friction in his experiments. Both sides found arguing about this difficult, because neither had a modern (Newtonian) concept of force. But Galileo suggested an experiment, shown in Fig 2.A2, in which a pendulum bob is let go but then the string is caught by a nail or peg. How would this provide more convincing experimental support for his theory?