FSS - Gravity Lesson

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Gravity

gravity is the attractive force between any two massesPhysics is the branch of science that investigates the matter and its motion, including forces.  Forces are simply any phenomenon that exerts a "push" or a "pull" on an object.   Some forces result from direct contact between two objects- a baseball bat exerts a force on a baseball (a push), and a truck exerts a force on a trailer that it is towing (a pull).   Other forces result from interactions between objects that are not in physical contact with each other.   These forces are known as "field" forces.   Field forces include magnetic, electrical, and gravitational forces; it is on the last of the three that we'll focus our attention on the most.

Gravity, as defined by Sir Isaac Newton, is an attractive force between any two masses.   We often think of gravity as a feature of planets and moons, but the truth of the matter is that every object has a gravitational attraction towards every other object.   You are exerting a gravitational pull on the computer in front of you, on the dog in your neighbor's yard, on a blue whale currently migrating in the Pacific Ocean,... everything!   But Newton also determined that not all pairs of objects exert the same gravitational attraction; combinations that result in smaller mass sums (between two ants, for example) do not generate as much pull as larger combined masses (between two trucks), and objects that are further apart do not exert as much pull as objects that are closer together (this is very similar to how magnets behave- as you bring a pair of magnets together, their attractive or repulsive force becomes stronger, while that same force becomes weaker as you increase the separation between the magnets).

In the presence of a large mass, such as the Earth, all of these tiny little gravitational forces between smaller objects are negligible; the relatively enormous pull that exists between the Earth and every object on its surface is so much stronger as to basically render these forces insignificant.   But, what if there was no pull from the Earth to mask these small forces?

In 1798, Henry Cavendish devised an experiment that allowed him to cancel out Earth's gravitational force on an object by adding another force, equal in magnitude but opposite in direction to the Earth's pull (it's really quite simple; he suspended a mass at the end of a string; gravity pulls down, the string pulls up, and the mass neither falls nor rises... brilliant!).   Now that Earth's gravity was effectively gone, Cavendish was able to investigate how these objects interact with each other.   What he discovered was that even small masses would pull themselves together in the absence of an overpowering gravitational force.   This is the foundation of the accretion theory of the origin of the solar system.  

Watch the presentation below to learn more.


 

Watch the video below to learn more about the basics of gravity and the Law of Universal Gravitation.

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