# Laws of Gravity: “Universal Gravitation” 1960 PSSC; Patterson Hume, Donald Ivey, U of Toronto

Physical Science Study Committee Films (PSSC) playlist:

Physics & Physical Sciences playlist:

“Explains the law of universal gravitation by imagining a solar system of one star and one planet. Professors Patterson Hume and Donald Ivey as inhabitants of planet X describe the process by which they derived the law in their solar system. Demonstrates the kinematics and dynamics of planetary motion using a large turntable as a model of a solar system. Displays satellite orbits as figured by a digital computer.”

Originally a public domain film, slightly cropped to remove uneven edges, with the aspect ratio corrected, and one-pass brightness-contrast-color correction & mild video noise reduction applied.

The soundtrack was also processed with volume normalization, noise reduction, clipping reduction, and/or equalization (the resulting sound, though not perfect, is far less noisy than the original).

Wikipedia license:

Newton’s law of universal gravitation states that a particle attracts every other particle in the universe using a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. This is a general physical law derived from empirical observations by what Isaac Newton called inductive reasoning. It is a part of classical mechanics and was formulated in Newton’s work Philosophiæ Naturalis Principia Mathematica (“the Principia”), first published on 5 July 1687. (When Newton’s book was presented in 1686 to the Royal Society, Robert Hooke made a claim that Newton had obtained the inverse square law from him; see the History section below.)

In modern language, the law states: Every point mass attracts every single other point mass by a force pointing along the line intersecting both points. The force is proportional to the product of the two masses and inversely proportional to the square of the distance between them. The first test of Newton’s theory of gravitation between masses in the laboratory was the Cavendish experiment conducted by the British scientist Henry Cavendish in 1798. It took place 111 years after the publication of Newton’s Principia and approximately 71 years after his death.

Newton’s law of gravitation resembles Coulomb’s law of electrical forces, which is used to calculate the magnitude of the electrical force arising between two charged bodies. Both are inverse-square laws, where force is inversely proportional to the square of the distance between the bodies. Coulomb’s law has the product of two charges in place of the product of the masses, and the electrostatic constant in place of the gravitational constant.

Newton’s law has since been superseded by Albert Einstein’s theory of general relativity, but it continues to be used as an excellent approximation of the effects of gravity in most applications. Relativity is required only when there is a need for extreme precision, or when dealing with very strong gravitational fields, such as those found near extremely massive and dense objects, or at very close distances (such as Mercury’s orbit around the Sun)…