Chapter 05

# Chapter 05 - Note that the following lectures include...

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Newton, Einstein, and Gravity Chapter 5
Astronomers are gravity experts. All of the heavenly motions described in the preceding chapters are dominated by gravitation. Isaac Newton gets the credit for discovering gravity, but even Newton couldn’t explain what gravity was. Einstein proposed that gravity is a curvature of space, but that only pushes the mystery further away. “What is curvature?” we might ask. This chapter shows how scientists build theories to explain and unify observations. Theories can give us entirely new ways to understand nature, but no theory is an end in itself. Astronomers continue to study Einstein’s theory, and they wonder if there is an even better way to understand the motions of the heavens. The principles we discuss in this chapter will be companions through the remaining chapters. Gravity is universal. Guidepost

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I. Galileo and Newton A. Galileo and Motion B. Newton and the Laws of Motion C. Mutual Gravitation II. Orbital Motion A. Orbits B. Orbital Velocity C. Calculating Escape Velocity D. Kepler's Laws Re-examined E. Newton's Version of Kepler's Third Law F. Astronomy After Newton III. Einstein and Relativity A. Special Relativity B. The General Theory of Relativity C. Confirmation of the Curvature of Space-Time Outline
A New Era of Science Mathematics as a tool for understanding physics

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Isaac Newton (1643 - 1727) Building on the results of Galileo and Kepler Major achievements: 1. Invented Calculus as a necessary tool to solve mathematical problems related to motion Adding physics interpretations to the mathematical descriptions of astronomy by Copernicus, Galileo and Kepler 2. Discovered the three laws of motion 3. Discovered the universal law of mutual gravitation
Velocity and Acceleration Acceleration ( a ) is the change of a body’s velocity ( v ) with time (t): 1. Acceleration in the conventional sense (i.e. increasing speed) a = v / t Different cases of acceleration: Velocity and acceleration are directed quantities (vectors)! 3. Change of the direction of motion (e.g., in circular motion) 2. Deceleration (i.e. decreasing speed) a v

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Acceleration of Gravity Acceleration of gravity is independent of the mass (weight) of the falling object! Iron ball Wood ball
Newton’s Laws of Motion (1) 1. A body continues at rest or in uniform motion in a straight line unless acted upon by some net force. An astronaut floating in space will continue to float forever in a straight line unless some external force is accelerating him/her.

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Newton’s Laws of Motion (2) 2. The acceleration a of a body is inversely proportional to its mass m, directly proportional to the net force F , and in the same direction as the net force.
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