Lecture5 - Making Sense of the Universe: Understanding...

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1 Making Sense of the Universe: Understanding Gravity If I have seen farther than others, it is because I have stood on the shoulders of giants .” Sir Isaac Newton (1642 – 1727) Review of Newton’s Laws of Motion First law : An object moves at constant velocity unless a net force acts to change its speed or direction. Second law: Force = mass acceleration Third law: For every force, there is always an equal and opposite reaction force. The Acceleration of Gravity • All falling objects near the Earth’s surface accelerate (pick up speed) at the same rate ( not counting friction of air resistance ). • We call this constant acceleration, g . • On Earth, g 10 m/s 2 . Speed increases 10 m/s with each second of falling. [A more exact number is 9.8 m/s 2 ] The Acceleration of Gravity ( g ) • Galileo showed that g is the same for all falling objects, regardless of their mass . A hammer and a feather if dropped at the same time should pick up speed at the same rate ! • Although true, it is hard to demonstrate on the Earth because the feather is affected by air resistance. • On the Moon, there is no air and this classic experiment was carried out on live TV by Apollo astronauts DEMO
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2 Gravitational Potential Energy • Energy can be stored by gravity • On Earth, gravitational potential energy depends on: – object’s mass (m) – strength of gravity ( g ) – distance object could potentially fall (h) P = mgh K=½mv 2 Gravitational Potential Energy • In space, an object or giant gas cloud has more gravitational energy when it is spread out than when it contracts. A contracting cloud converts gravitational potential energy to kinetic energy and thus thermal energy. Conservation of Energy
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This note was uploaded on 02/20/2012 for the course ASTRO 3 taught by Professor Mclean during the Spring '11 term at UCLA.

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Lecture5 - Making Sense of the Universe: Understanding...

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