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Lect04 newton's laws - Physics 211 week 3 Todays Agenda q q...

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Physics 211: Lecture 4, Pg 1 Physics 211: week 3 Physics 211: week 3 Today’s Agenda Today’s Agenda Recap of centripetal acceleration Newton’s 3 laws (these describe all of classical mechanics!) How and why do objects move? Dynamics Dynamics Original: Mats A. Selen Modified: W.Geist
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Physics 211: Lecture 4, Pg 2
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Physics 211: Lecture 4, Pg 3 Forces? Forces? We will consider 2 kinds of forces: 1.Contact forces (normal force, force of string on ball F B,S etc. These forces are identified by the contact between the object under consideration (the ball) and the objects that are in contact with this object (the string). 2. Long range forces: The only long range force in this class is the force of gravity. Gravitational force of the earth on the student.
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Physics 211: Lecture 4, Pg 4 Dynamics Dynamics Isaac Newton (1643 - 1727) published Principia Mathematica in 1687. In this work, he proposed three “laws” of motion: 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 (here we compare (here we compare two different concepts and combine them with an equation) two different concepts and combine them with an equation) 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 5 Newton’s First Law Newton’s First Law 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 . If no forces act, there is no acceleration. The following statements can be thought of as the definition of inertial reference frames. An IRF is a reference frame that is not accelerating (or rotating) with respect to the “fixed stars”. If one IRF exists, infinitely many exist since they are related by any arbitrary constant velocity vector! If you can eliminate all forces, then an IRF is a reference frame in which a mass moves with a constant velocity. (alternativedefinition of IRF )
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Physics 211: Lecture 4, Pg 6 Is Atlanta a good IRF? Is Atlanta a good IRF? Is Atlanta accelerating? YES! Atlanta is on the Earth. The Earth is rotating. What is the centripetal acceleration of Atlanta? T = 1 day = 8.64 x 10 4 sec, R ~ R E = 6.4 x 10 6 meters . Plug this in: a U = .034 m/s 2 ( ~ 1/300 g) Close enough to 0 that we will ignore it. Atlanta is a pretty good IRF. a v R R T R U = = = 2 2 2 ϖ π 2
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Physics 211: Lecture 4, Pg 7 Newton’s Second Law Newton’s Second Law For any object, F NET = Σ F = m a .
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