From the apple that famously fell on Newton's head, inspiring the laws of motion and universal gravitation, to Einstein's revelation of gravity as the warping of space-time by mass and energy, our understanding of gravity has evolved dramatically.
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So although science today sees gravity as a distortion of spacetime, rather than a force, nonetheless, Newton's understanding of gravitation is an appropriate model, relevant to most situations :
it's still is good enough to get us to the moon and back. These series of lessons will cover of the classical understanding of gravitation, and its consequences, as developed by Isaac Newton. |
Law of Gravitation
This video covers the Law of Gravitation, as Newton understood it.
What is also important as this is an example of a unification of ideas or theories.
The concept of unification in science is to develop an overarching model that explains seemingly two separate models. In the case of gravitation, Newton came to the conclusion that the force of gravity that caused objects to fall to the ground was the same force that was responsible for keeping planets in orbit. Prior to Newton's work these were considered a separate ideas.
As you watch the video below, you will see that the law of gravitation is another example of an inverse square law. It is important to realise, however, that what Newton developed was not an explanation of what causes gravity, rather a description of how gravity behaved. A fuller explanation of what caused gravity did not develop until Einstein's General theory of relativity (GR) in the early part of the 20th century.
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As you watch the video, you will see that the law of gravitation is another example of an inverse square law.
It is important to realise, however, that what Newton developed was not an explanation of what causes gravity, rather a description of how gravity behaved. A fuller explanation of what caused gravity did not develop until Einstein's General theory of relativity (GR) in the early part of the 20th century. |
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Interactive
A you can use this interactive is to set the masses to a predefined distance and then calculate the strength of the force due to gravitation. You then compare the result with the quoted value
Sample Problem
We are now ready to try a sample problem
Below is a sample problem with a video that explain how to solve it. It is suggested you try the problem beforehand, as this actually aids understanding, even if you are unsure if you are correct.
We are now ready to try a sample problem
Below is a sample problem with a video that explain how to solve it. It is suggested you try the problem beforehand, as this actually aids understanding, even if you are unsure if you are correct.
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Here are some more problems for you to try. Answers are provided.
- The radius of the earth is about 6400 km. What would be the earths gravitational attraction on a 75 kg astronaut in an orbit of 6400 km above the earth surface? (183.75N)
- The mass of Mars is about 6.6×1023 kg, and the acceleration due to gravity is 3.7 ms-2. What is the radius of Mars? (3450km)
Two objects with the same mass are placed 60 cm apart. If the gravitational force between the objects is 7×10-9 N, what is the mass of each object? (6.15 kg) - At what altitude above earth's surface is the acceleration due to gravity be 4.9 ms-2? (2670km)
- A sphere of mass 85 kg is 12 m from a second sphere of mass 65 kg. What is the gravitational force of the attraction between them? (1.01 x 10-8 N)
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Going Deeper - Gravity Variation |
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