Gravity

# Gravity - Mechanics and Modern Physics Dr Bernd Stelzer...

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Dr. Bernd Stelzer Mechanics and Modern Physics PHYS 120 SFU Fall 2010 Dr. Bernd Stelzer

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Announcements November 22, 2010 Bernd Stelzer PHYS120 2 Mastering Physics (MP4) online assignment is available from www.masteringphysics.com - Due next Monday (Nov 22 nd , 2010) - Recommended browser is: Firefox This is a ‘Quiz week’ for tutorials Space available in the extra tutorials (P8445.2) - Great resource to prepare for final exam - Small groups discussing your questions! Monday 2:30-3:20 PM Geoff Archibald Tuesday 3:30-4:20 PM Sarah Johnson Thursday 2:30-3:20 PM Geoff Archibald Friday 11:30 AM-12:20 PM Sarah Johnson
Survey Out of 18 points total, I expect to have scored in the range: A. 0 - 5 points B. 6 – 9 points C. 10 – 12 points D. 13 – 15 points E. 16 – 18 points 11/22/10 Bernd Stelzer PHYS120 3

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Midterm 1 - Results 11/22/10 Bernd Stelzer PHYS120 4 Average: 12.1 / 18
Newton’s Theory of Gravity 11/22/10 Newton proposed that every object in the universe attracts every other object following a simple Law. Goal: Using Newton’s theory of gravity to understand the motion of satellites and planets. PHYS120 5 Bernd Stelzer

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Newton’s Law of Gravity 11/22/10 Newton proposed the universal law of Gravity : 1. The force is inversely proportional to the square of the distance between the objects. 2. The force is directly proportional to the product of the masses of the two objects. PHYS120 6 Bernd Stelzer F 1on2 F 2on1
Gravitational Attraction Force 11/22/10 Let’s calculate the gravitational force between two people: F 12 = F 21 = Gm 1 m 2 r 2 = 6.67 10 11 N m 2 / kg 2 50 kg 60 kg (1 m ) 2 = 2.0 10 7 N In comparison, the maximum static friction force on, e.g. m 1 is: f s max = μ s n = μ s mg = 1 50 kg 9.80 m / s 2 = 490 N Since f s max >> F 21 it is clear why we do not accelerate towards each other! This is tiny! PHYS120 7 Bernd Stelzer

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Gravitational Attraction Force 11/22/10 Let’s replace the second person by the earth. ! F 12 = ! F 21 = Gm 1 m 2 r 2 = 6.67 " 10 # 11 N " m 2 / kg 2 " 50 kg " 5.98 " 10 24 kg (6.37 " 10 6 m ) 2 = 491.5 N Gravitational forces are negligible unless planet size objects are involved This is larger! PHYS120 8 Bernd Stelzer