Forces - University of California, San Diego Center for...

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University of California, San Diego Gene Smith's Astronomy Tutorial NEWTON'S LAWS OF MOTION Here's another look at Newton's Laws .
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Newton's First Law -- Inertia Unless a body is acted upon by an outside influence (force), it will remain in uniform motion. A body at rest will remain at rest; a moving body will continue moving at the same speed in the same direction. Here is another look at Newton's First Law . Newton's Second Law -- F = ma Mass is the measure of the inertia of the body - how hard it is to get the body moving (grams). Force is the outside influence (dynes - like our Chancellor - in cgs units). Acceleration is the change in motion; this can be speeding up, slowing down, or changing direction (cm/s 2 ) Another look at Newton's Second Law . Newton's Third Law -- When body A exerts a force on body B, body B exerts an equal and opposite force on body A. F A B = - F BA Often stated as the Law of Action and Reaction, the Third Law is responsible for Conservation of Momentum and is the Principle that makes Rocket Flight possible. Another look at Newton's Third Law . THE FUNDAMENTAL FORCES As far as we know, there are four Fundamental Forces : Gravity Weak Nuclear Force Electromagnetism (or Electricity) The Strong Nuclear Force in increasing order of strength. For another look at the fundamental forces follow this link. Gravity - The First Fundamental Force Newton knew from his Third Law that the Force of Gravity must be reciprocal between the two masses involved. By applying his newly developed mathematical tool called fluctions ( i.e. calculus), to the orbit of the Moon around the Earth, he was able to determine that the force must depend
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upon the inverse square of the distance between the Earth and the Moon. The gravitational force between the Earth and the Moon, or any two masses, is given by where is the mass of the Earth, and is the mass of the Moon, d is the distance between the Earth and the Moon, and G is Newton's Gravitational constant (G = 6.67 x 10 - 8 dyne . cm 2 /g 2 ) Gravity is such a weak force that Newton's constant could not be measured at that time. The first to estimate the value of G was astronomer Nevil Maskelyne . He used the deflection of a plumbline on the slopes of Mt. Schiehallion in Scotland to determine the gravitational attraction between the plumbob and the mountain in 1774. From Schiehallion's very regular shape,
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This note was uploaded on 01/12/2012 for the course PHYSICS 4B taught by Professor Eduardoluna during the Winter '11 term at DeAnza College.

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Forces - University of California, San Diego Center for...

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