Weighing our planet may seem like an almost impossible task. However, the question that has puzzled scientists for ages is “How much does the Earth weigh?” It took until 1687 to find the answer to the question, how to weigh (measure its mass) the Earth, and even then we were not able to calculate this value until 1798.
In the late 1600s, Newton proposed the law of universal gravitation: Every particle in the universe attracts all other particles in the universe with a force (F) determined by their mass (M) and the square of the distance (R) between the centers of objects: F=G(M1xM2/D2).
As the formula shows, if you have the mass of one of the objects and all the other information in the equation, you can calculate the mass of the second object. Assuming you are one of the known masses, you can calculate the weight of the Earth since we roughly know how far we are from the center of the Earth. But the problem here was that in Newton’s time we didn’t have a value for G, and so the equation couldn’t be solved.
Newton thought that measuring the gravitational force of an object was not possible for objects smaller than planets and moons. However, this was not the case. Knowing the mass and density of the Earth would be incredibly useful to astronomers. It was interesting data to know on its own, as it would help them calculate the mass and density of other objects in the Solar System. In 1772, the Royal Society formed a “Committee of Attraction” to resolve this.
Shooting Committee
Attempts have been made to measure the average density of the Earth using a mountain in Scotland. The team showed that Schiehallion’s enormous mass pulls the pendulums towards it. By measuring the movement of the pendulum and examining the mountain, they were able to roughly calculate the density of the Earth.
But they finally found the value of G in 1797 and we were able to calculate the mass of the Earth. Geologist Reverend John Michell was working on this problem, but was unable to finish his work before his death. Instead, scientist Henry Cavendish, who used Michell’s equipment to carry out the experiment, took on the task.
Using a relatively simple setup, Cavendish was able to measure the force between two metal spheres separated by a known distance. The gravitational force exerted by the earth on the smaller ball could be measured by weighing it, and the density of the balls was also known.
If you’re interested, looking at the ratio between the two forces revealed that the mass of the Earth is about 5,974,000,000,000,000,000,000 kilograms.
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