# Lect03 - PHYS 172: Modern Mechanics Lecture 3 - The...

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Fall 2011 PHYS 172: Modern Mechanics Lecture 3 - The Momentum Principle Read 2.1-2.6 1

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TODAY •Relativity •Momentum Principle •Impulse •Net Force •Predicting Motion 2
RELATIVITY l Physical laws work in the same way for observers in uniform motion as for observers at rest. z avg v v The position update formula ( ) f i avg f i r r v t t = + ! ! ! ! (=in all inertial reference frames) 3

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l Physical laws work in the same way for observers in uniform motion as for observers at rest. z The position update formula ( ) f i avg f i r r v t t = + ! ! ! ! ! v avg ! v avg (=in all inertial reference frames) Note: all parameters must be measured in respect to the selected reference frame to predict motion in respect to that reference frame RELATIVITY 4
The Momentum Principle ! p = ! m ! v = 1 1 ! ! v c " # \$ % & ' 2 An object moves in a straight line and at constant speed except to the extent that it interacts with other objects The Momentum Principle ! ! p = ! F net ! t Change of momentum is equal to the net force acting on an object times the duration of the interaction • Assume that F does not change during Δ t What is l force z F ? • measure of interaction. • defined by the momentum principle. 2 F units: N m kg/s ≡⋅ (Newton) 5

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Impulse The Momentum Principle ! ! p = ! F net ! t Definition of impulse Impulse ! ! F net " t Momentum principle : The change of the momentum of an object is equal to the net impulse applied to it Note: small Δ t F net ~ const 6
The principle of superposition !

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## This note was uploaded on 12/07/2011 for the course PHYS 172 taught by Professor ? during the Fall '08 term at Purdue University-West Lafayette.

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Lect03 - PHYS 172: Modern Mechanics Lecture 3 - The...

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