What pulls, and what holds?

This lesson asks you to give examples of different forces and to describe the effect of each on an object's velocity. The museum offers one historical description of attraction, a medieval author's explanation of why objects fall towards the Earth. Friction, normal force and the effects on velocity are for your class to bring. ¹

Look at the expectation: it asks for examples of forces and their effect on velocity. For the description, ask what is pulling, what is being pulled, and whether it says anything about the object's speed or direction.

Read one comparison in a preface. TDV records a physical explanation in al-Idrisi's preface: he compares the earth's attraction of objects with a magnet attracting iron, explaining why airborne objects fall towards the earth ². The book itself begins with the shape of the earth. The earth, al-Idrisi writes, is round like a ball, with the water clinging to it, and earth and water rest in the middle of the heavens like the yolk in an egg ³. The museum attributes the magnet analogy to him rather than presenting a modern gravitational law or experimental discovery ². The comparison is part of the explanation of the world that comes before the book's maps and distances ².

Al-Idrisi's preface tells the story of the work as the story of the king. Roger, he writes, was learned in the mathematical sciences, and when his realm had grown he wanted to know its lands with certainty ⁴. The books did not satisfy him, so he questioned those who knew the subject and then the travellers, keeping what they agreed on and setting aside what they disputed, for about fifteen years ⁵. In the preface it is Roger who drives the inquiry, and al-Idrisi presents himself as the one who carried out his order ⁶. The records give no purpose of al-Idrisi's own beyond that order.

A comparison is not a law. The museum attributes the magnet analogy to al-Idrisi and does not present it as a modern gravitational law or an experimental discovery ². The record adds a second caution: a spherical earth and a correct distribution of climates and habitation are separate claims, and accepting the former does not make every regional description accurate ².

In the record, the magnet comparison rests on TDV's reading of the preface, while the story of Roger's inquiry and the shape of the earth come from the Arabic preface, in the Cairo edition of 2002 ^{2 7}. TDV also notes the limits of the book: its reports on the lands of the Turks are fuller than those of earlier geographers but at times blurred by legend ⁸. The sources read for the record say nothing of what became of the silver disc. Ask which statements about the falling of objects you could check in al-Idrisi's own words, and which you receive through a summary.

Does comparing the earth's pull to a magnet's pull explain why objects fall, or only name it, and what does the comparison leave unsaid about how a falling object's speed or direction changes?

• Al-Idrisi's world map and book: the silver disc, the seventy maps and the preface.
• Al-Idrisi: his life at Palermo and what TDV says of his works.
• Nuzhat al-mushtaq: the book, its sections and its names.
• The seven climes: the division of the inhabited world his book follows.

Read the record of al-Idrisi's world map and book. Name the force described and draw an arrow for it on a sketch of a falling object, then add what the record does not say: how the speed or direction changes.