Gravity isaac newton5/5/2023 ![]() ![]() You can calculate that the surface of the Earth falls below a flat horizontal line by approximately 5 m over a distance of 8 km. Let us first treat this experiment quantitatively and use modern day values and units to extend Newton’s arguments to encompass the Moon. Newton’s thought experiment using a cannon ball. One can see from this that, if the velocity is gradually increased there would come a point when the cannon ball would never land - and would be in an orbit around the Earth. However, this is a thought experiment, not a real one. The mountain is impossibly high in order for it to reach above the Earth’s atmosphere. In a more popular work published in the 1680s called A Treatise of the System of the World. As the landing point becomes further away the curvature of the Earth must be considered. As the cannon ball was fired with greater and greater velocity it would land further and further away from the mountain. The cannon ball would follow a parabolic path to the ground. Newton’s moment of genius was to realise that the Moon was falling towards the Earth! He was aware of Galileo’s work relating to the trajectories of projectiles and, in his great work Principia published in 1686, he considered what would happen if one fired a cannon ball horizontally from the top of a high mountain where air resistance could be ignored. It is said that an apple dropped on his head (or thudded to the ground beside him) and this made him wonder why the Moon did not fall towards the Earth as well. He might well have been able to see the first or last quarter Moon in the sky. There is a story (which is probably apocryphal) that Newton was sitting under the apple tree in the garden of Woolsthorpe Manor. It was there, in a period of 2 years and whilst he was still under 25, that his genius became apparent. He received his bachelor’s degree in April 1665 but then had to return home when the University was closed as a result of the Plague. He lodged with the school’s headmaster who gave Isaac private tuition and he was able to enter Trinity College, Cambridge in 1661 as a somewhat more mature student than most of his contemporaries. An uncle persuaded his mother that Isaac should prepare for entering university and so, in 1660, he was allowed to return to the Free Grammar School in Grantham to complete his school education. His mother later took Isaac away from school to manage her property and land, but he soon showed that he had little talent and no interest in managing an estate. He attended the Free Grammar School in Grantham but showed little promise in academic work and his school reports described him as ‘idle’ and ‘inattentive’. Isaac was left in the care of his grandmother and effectively treated as an orphan. His father, also called Isaac Newton, had died before his birth and his mother, Hannah, married the minister of a nearby church when Isaac was 2 years old. Isaac Newton was born in the manor house of Woolsthorpe, near Grantham, in 1642, the same year Galileo died. Isaac Newton and his Universal Law of Gravity ![]()
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