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Lect04-Phys172s10-(2.7-2.8,3.4,3.11-Conservation_of_momentum)

Lect04-Phys172s10-(2.7-2.8,3.4,3.11-Conservation_of_momentum)

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Spring 2009 PHYS 172: Modern Mechanics Lecture 4 – Conservation of Momentum Read 2.7–2.8, 3.4, 3.11 Physical models “Spherical cow” Ideal model: ignore factors that have no significant effect on the outcome

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Example: colliding students Two students are late for class and run into each other head-on. Q: Estimate the force that one student exerts on the other during collision Simplest model: System: one spherical student Surroundings: earth, floor, air, second spherical student Force: Earth F ° , floor N F ° air F ° , floor P F ° F ° Earth, floor, air, other student – unknown! Example: colliding students f i net p p F t - = Δ ° ° ° f i avg r r v t = + Δ ° ° ° Strategy: p mv γ = ° ° f i net p p F t - = Δ ° ° ° , , 0,0 ,0 , ix floor P air floor N Earth p F F F F F t < > - < >=< - - - > Δ Earth F ° , floor N F ° air F ° F ° , floor P F ° x y ,0 ,0 ix p F t < - >=< - > Δ ix p F t = Δ
Example: colliding students f i net p p F t - = Δ ° ° ° f i avg r r v t = + Δ ° ° ° Strategy: p mv γ = ° ° Earth F ° , floor N F ° air F ° F ° , floor P F ° x y ix p F t = Δ Assume: v i =5 m/s, Δ x =0.05m avg x v t Δ = Δ avg x t v Δ Δ = 0.02 s t Δ = What is the collision time? What is the initial momentum?

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