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# lec14 - Physics 140 Fall 2007 lecture#14 23 October Ch 8...

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Announcements: exam #2 is next Thursday, 1 November, 6:00-7:30pm • covers Chapters 6-8 • practice exam on course web page under Resources link review next Monday evening, 29 Oct 182 Dennison 8:00-9:30pm Physics 140 – Fall 2007 lecture #14: 23 October Ch 8 topics: • center of mass • rockets One should not pursue goals that are easily achieved. One must develop an instinct for what one can just barely achieve through one’s greatest efforts. The New Quotable Einstein , A. Calaprice, ed., Princeton Univ. Press, 2005, p. 220.

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For a pair of objects, one of mass m 1 located at r 1 and the other of mass m 2 located at r 2 , we define the center of mass position by a mass-weighted, normalized sum If the objects have velocities v i and accelerations a i (i=1,2), we similarly define the center of mass velocity as and center of mass acceleration as r com = ( m 1 r 1 + m 2 r 2 ) / ( m 1 + m 2 ) . v com = ( m 1 v 1 + m 2 v 2 ) / ( m 1 + m 2 ) a com = ( m 1 a 1 + m 2 a 2 ) / ( m 1 + m 2 ) . Center of mass: position, velocity and acceleration Generalize to n masses by extending the sums v com = 1 M m i v i i = 1 n " r com = m i r i i = 1 n " m i i = 1 n " = 1 M m i r i i = 1 n "

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lec14 - Physics 140 Fall 2007 lecture#14 23 October Ch 8...

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