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Lect05-Phys172s10-(3.1-3.5-Fundamental_forces)

# Lect05-Phys172s10-(3.1-3.5-Fundamental_forces) - PHYS 172...

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1 Spring 2010 PHYS 172: Modern Mechanics Lecture 5 – Fundamental Forces Read 3.1 – 3.5 The Four Fundamental Forces “Composite” forces like the spring force, air drag, friction, etc. are combinations of these four fundamental forces

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2 Newton’s Great Insight: The force that attracts things toward the earth (e.g. a falling apple) is the same force that keeps planets orbiting about the sun Question: Why doesn’t the Moon fall into the Earth? The gravitational force law 2 1 on2by1 2 1 2 2 1 ˆ grav m m F G r r - - = - ° ° m 1 m 2 2 1 r - ° 2 1 ˆ r - Newton 2 1 2 1 r r r - - ° ° ° m 1 m 2 2 r ° 1 r ° 2 11 2 N×m 6.7 10 kg G - = × Cavendish Gravitational constant
3 Features of gravitational force 2 1 on 2by1 2 1 2 2 1 ˆ grav m m F G r r - - = - ° ± gravity is always attractive 2 1 on 2by1 2 1 2 2 1 ˆ grav m m F G r r - - = - ° ± gravity is an inverse square law 2 1 on 2by1 2 1 2 2 1 ˆ grav m m F G r r - - = - ° ± the force depends upon the product of the masses Distance between two objects Point object: idealized object which has no size, all mass is in one point If distance between the two objects is >> than their size, can model the objects as point-masses Real objects: have size

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