Lect04

# Lect04 - PHYS 172 Modern Mechanics Fall 2011 Reminders 1...

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Fall 2011 PHYS 172: Modern Mechanics Lecture 4 – Physical Models, Fundamental Interactions Read 2.7–2.8, 3.1-3.4 Reminders: 1. Please register your iClicker on CHIP a.s.a.p. We plan to transfer your iClicker scores to CHIP next week. 2. Homework due date extensions can be only requested before the official due date. 1

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Today •Poor Monkey •Reciprocity (Equal and Opposite Forces) •Example: Colliding Students •Four Fundamental Forces •Gravity Lots of Gravity 2
Reciprocity: Newton ` s 3d law ! F spring on mass Force magnitudes are the same Directions are opposite They act on different objects Reciprocity (Newton ` s 3 rd law): The forces of two objects on each other are always equal and are directed in opposite directions ! F mass on spring ! F spring on mass = ! ! F mass on spring ! ! p = ! F net ! t NOTE: Velocity-dependent forces (e.g., magnetic forces) do not obey Newton ` s 3 rd law! 3

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Physical models l Spherical cow z Ideal model: ignore factors that have no significant effect on the outcome 4
Example: colliding students Mary and John 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: Mary John !

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Lect04 - PHYS 172 Modern Mechanics Fall 2011 Reminders 1...

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