# Lect04 - Physics 211 Lecture 4 Todays Agenda Recap of...

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Physics 211: Lecture 4, Pg 1 Physics 211: Lecture 4 Physics 211: Lecture 4 Today Today ’s Agenda s Agenda z Recap of centripetal acceleration z Newton’s 3 laws (these describe all of classical mechanics!) ¾ How and why do objects move? ¾ Dynamics Dynamics

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Physics 211: Lecture 4, Pg 2
Physics 211: Lecture 4, Pg 3 Dynamics Dynamics z Isaac Newton (1643 - 1727) published Principia Mathematica in 1687. In this work, he proposed three “laws” of motion: z principia Law 1 : An object subject to no external forces is at rest or moves with a constant velocity if viewed from an inertial reference frame. Law 2 : For any object, F NET = Σ F = m a a Law 3 : Forces occur in pairs: F A ,B = - F B ,A (For every action there is an equal and opposite reaction.) These are the postulates of mechanics They are experimentally, not mathematically, justified. They work, and DEFINE what we mean by “forces”.

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Physics 211: Lecture 4, Pg 4 Newton Newton ’s First Law s First Law z An object subject to no external forces is at rest or moves with a constant velocity if viewed from an inertial reference inertial reference frame frame . ¾ If no forces act, there is no acceleration. z The following statements can be thought of as the definition of inertial reference frames. ¾ An IRF is a reference frame that is not accelerating (or rotating) with respect to the “fixed stars”. ¾ If one IRF exists, infinitely many exist since they are related by any arbitrary constant velocity vector! ¾ If you can eliminate all forces, then an IRF is a reference frame in which a mass moves with a constant velocity. (alternative definition of IRF ) 1. Dishes 2. Monkey
Physics 211: Lecture 4, Pg 5 Is Urbana a good IRF? Is Urbana a good IRF? z Is Urbana accelerating? z YES! ¾ Urbana is on the Earth. ¾ The Earth is rotating. z What is the centripetal acceleration of Urbana? ¾ T = 1 day = 8.64 x 10 4 sec, ¾ R ~ R E = 6.4 x 10 6 meters . z Plug this in: a U = .034 m/s 2 ( ~ 1/300 g) z Close enough to 0 that we will ignore it. z Urbana is a pretty good IRF. a v R R T R U == = 2 2 2 ω π 2 Ice puck

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Physics 211: Lecture 4, Pg 6 Newton Newton s Second Law s Second Law z For any object, F NET = Σ F = m a . ¾ The acceleration a of an object is proportional to the net force F NET acting on it. ¾ The constant of proportionality is called “mass” , denoted m . » This is the definition of mass and force . » The mass of an object is a constant property of that object, and is independent of external influences. » The force is the external influence » The acceleration is a combination of these two things z Force has units of [ M ]x[ L / T 2 ] = kg m/s 2 = N (Newton)
Physics 211: Lecture 4, Pg 7 Newton Newton ’s Second Law. ..

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Lect04 - Physics 211 Lecture 4 Todays Agenda Recap of...

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