GravitationIntroduction
A cricket ball hit high into the air always comes back down. Rain falls from the clouds, the mango drops from the tree, and walking uphill is harder than walking down. We learn very early that everything is pulled towards the Earth. Galileo went further: he showed that, if air does not get in the way, every body falls with the same acceleration, however heavy it is.
Up on the roof at night, Sina points at the sky. The stars keep their places, year after year, but a few bright "stars" wander slowly against them. These wanderers are the planets, and explaining their motion took people about two thousand years. Ptolemy put the Earth at the centre with everything circling it (the geocentric model), and needed circles riding on circles to match what was seen. Aryabhata, in the fifth century, already wrote of a model with the Sun at the centre (the heliocentric model), and Copernicus revived it about a thousand years later. Tycho Brahe then spent his life measuring planet positions with the naked eye, and his assistant Kepler found three neat laws hidden in that data. Newton used those laws to discover the law of gravitation, which explains the falling mango and the wandering planets together.
Put these people in the order in which they lived.
Ptolemy (about 2000 years ago), Aryabhata (fifth century), Copernicus (1473–1543), Tycho Brahe (1546–1601), Kepler (1571–1630) and Newton (1642–1727). This chapter follows the same road: Kepler's laws first, then Newton's law and the constant G in it, then what the law says about g above and below the Earth's surface, the energy of a body in the Earth's pull, and finally escape and satellites.