Lecture_12

Lecture_12 - P123Lecture1219Nov2010...

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P123 Lecture 12 19 Nov 2010 CENTER OF MASS MOTION & ROTATIONAL KINEMATICS REVIEW • LINEAR MOMENTUM: v m p dt p d a m F • IMPULSE OF A FORCE: (Strong force acting for short time) ) ( 1 2 t t F J F ) ( 1 2 2 1 t t F dt F J t t x x x t x F x • Impulse-Momentum Theorem • CONSERVATION OF LINEAR MOMENTUM: 1 2 P P J t 1 t 2 • TYPES OF COLLISIONS: Elastic: conserved then 0 If I F ext P P F ] [ ] [ F F I I B A B A P P P P KE P & Inelastic: only is conserved Completely Inelastic: only conserved objects stick P P 1 • LINEAR MOMENTUM CONSERVED IN COLLISIONS (When collision force >> external force)
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i Clicker Many nuclear reactors use Li ( M = 7 m o ) to moderate ( reduce the KE ) of neutrons ( M n = m o ) Li n produced in nuclear reactions. An alternative is sodium ( M Na = 23 m o ). Na n - or - Li is a better choice because: A. P tot is lower after n Li collision B. n has less KE after n Li collision compared to n Na collision C. P tot is higher after n Na collision D. KE of n does not change after n Na collision E. Na is better choice (trick question) 2 P final = P initial
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CENTER OF MASS Point in an extended mechanical system that moves as though all the mass were concentrated at that point Consider a collection Consider a collection of (different) masses distributed on x-axis. m m m Define center of mass: ... ... 3 2 1 3 3 2 2 1 1 m m m x x x x cm m m i.e.: Similar expression for and M x m x x i i i i i cm i i i Similar expression for cm y cm z r m r i i cm i If mass is a continuous distribution: M 3 dm r r M cm 1
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EXAMPLE : m m m m x cm 3 ) 2 ( 3 ) 4 ( 2 ) 6 4 1.5 4.5 x 2 1 4 4 )) ( ( ml l m 1 1 3 = 6 x x 1 3 3 x x 1 4 1 4 4 l 4
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This note was uploaded on 03/14/2011 for the course P 123 taught by Professor Stevens during the Spring '11 term at Rust.

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Lecture_12 - P123Lecture1219Nov2010...

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