Sir Isaac Newton

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Physics, Engineering Physics and Astronomy
ASTR 101

Sir Isaac Newton The First Law: Inertia Formalized • Any object in a state of motion (including rest) will remain in that state unless some unbalanced force is acting  Assumed the force is unbalanced as changes in state usually do not occur if it is balanced  Counters Galileo's understanding of Inertia as the Moon's orbit was not caused by natural coasting with no acting forces, as Gravity was acting upon it  Space stations work in similar ways by remaining in Earth's gravity, their orbit is due to gravity as their initial direction of movement was a straight line The Second Law: Unbalanced Forces • Objects with more mass have greater inertia and need more force to be brought into motion or F = ma or a = F / m  F = amount of outside force on an object (unbalanced)  m = mass  a = acceleration (can be negative! = slowing down) • More external force on an object yields higher acceleration • Mass = the measure of total material in an object, not the size or weight  If you apply a large unbalanced force, you get a large acceleration  If the object has a large mass, a small amount of force will barely affect it The Third Law: Action and Reaction • Whenever one object exerts a force on another object, the second objects exhibits a reaction of equal and opposite force  Every action has an equal and opposite reaction (closely related to conservation of linear momentum)  Newton realized that because the planets are gravitationally attracted to the Sun, the Sun is equally attracted to the planets, but since the planets are much less massive, they have less effect on the sun  Example: When you do a push up, you are pushing into the ground with equal and opposite force, so your body moves up but a force downwards is sent into the Earth  Earth is so massive our actions have no affect on its movements Deterministic Physics • Theoretically, by knowing all of the acting forces we can determine where and how everything should move  Very difficult to know all of the acting forces  Example: a game of pool the player angles the shot to predict how the ball will move while accounting for air resistance and friction How Rockets Work • Rockets follow the law of linear momentum or Newtons third law: as the gas is sent out of the rocket at high speeds the rocket is launched in the opposite direction with equal force Newtonian Gravity • Gravity follows the inverse square law, meaning that if you move twice as far from the Sun than Earth is, you will feel 1/4 of the Sun's gravitational force compared to that on Earth  Newton assumed this law was universal, meaning it applied throughout space • To examine the cu
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