It was also possible to time how long it took the Moon to go around the Earth (called the moon's period, T). In Newton’s time man was able to measure the distance, d, to the Moon with geometry. There is another method of finding the mass of the Earth using Newton’s physics. (That’s 6 trillion-trillion kilograms!) Cavendish set out to find G, but he ended up finding the number by which we were able to "weigh" the Earth! We also know how far you are from the Earth’s center, d. Your weight is known, and it’s the force F that goes in Newton's equation. (G is known only to the six decimal places.)Īssume you are mass m and the Earth is Me. He carefully measured the force between them, which allowed him to calculate G. He knew the masses of the spheres and how far apart they were. In 1798 Cavendish measured the force between attracting lead spheres with a torsion balance. G is a constant that must be determined experimentally. This attractive force is directly proportional to the masses and inversely proportional to the square of the distance between them and is written F = G M m / d2, where F denotes the force of attraction, M and m are the masses, and d is the distance between the object’s centers. Newton’s Law of Universal Gravitation states that any two masses are attracted to one another. The first person credited with "weighing" the Earth was Henry Cavendish (1731-1810.)īut we must first mention one of the greatest physicists who ever lived - Isaac Newton (1642-1727). Too bad we can’t place the Earth on a big bathroom scale! The Earth’s mass must be found by indirect means. How did mankind first determine the mass of the Earth, and what is the known value today? June 2002 Vocatio Center for Life Calling and Career.Office of Student Leadership & Engagement.
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