force - Solomons Study Notes College Physics 1 Mechanics...

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Solomon’s Study Notes College Physics 1 Mechanics & Heat Fall 2011 Solomon Weiskop PhD [ Force ] These Notes cover Force (including Newton’s Laws)
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Study Notes are available to print out by registering at www.solomonlinetutor.com Solomon Weiskop PhD Solomon’s Tutoring © Copyright 2011
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1 1. Newton’s 2 nd Law (General Comments) In the Study Notes on Motion (1) , when discussing free fall, I referred to various “influences” on a baseball. The proper term is Force. A force acting on an object influences its motion. One may say that Motion is the “effect” and Force is its “cause”. To be more precise, according to Newton’s 2 nd Law (N2L): (N2L) a net force acting on an object (of mass ) causes it to have an acceleration . That is, force is the cause and acceleration is the effect . both force and acceleration are vectors N2L is a vector equation is just Vector Addition. You can have several forces acting on some object. To get the “net force” acting on that object you add up all these forces by vector addition. So is the “resultant force”. is just Multiplication of a Vector ( ) by an ordinary number ( ). Mass is always positive so we can conclude that and will be in the same direction . But that’s just math. Saying this in a more physically meaningful way: an object accelerates in the direction of the net force exerted on it Writing N2L in the form shows that mass is “inertia”. For a given net force, an object will accelerate less if its mass is bigger and will accelerate more if its mass is smaller. standard unit for mass is (kilogram) standard unit for force is (Newton) ( )
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2 2. A Few Common Forces Force is a vector: it has both magnitude and direction. When solving force problems, you ’ll encounter several common types of forces. Each of these common types of force has its own “ Direction R ule” which it obeys completely independently of whatever other forces there may be in the problem, or of whatever motion is occurring in the problem. Force Direction Rule Weight ( ) down Tension ( ) a string can pull (not push) Normal force ( ) perpendicular to surface Friction force ( ) opposes the motion (specifically ) Weight ( ) is a force. It is the gravitational pull on an object due to the Earth. Since gravity points down (and is positive) an object’s weight always points down. (Direction Rule) For now, we will only c onsider objects at or near the earth’s surface. Later on in this course, we will consider gravitational forces on objects that are at a distance away from Earth’s surface. Tension in a string is also a force. You know from experience that you can only pull an object with a string. You cannot push an object with a string. (Direction rule) When you draw a vector that represents tension (in a string) acting on an object: must be in the string and it must point away from the object (i.e. it must pull , not push, the object) One String → One Tension The tension throughout a string has the same magnitude, even if that string is looped over a pulley (or pulleys). Of course, different
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