Lecture5 - Physics 41 Lecture 5 Lecture 5 Force Newton’s...

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Unformatted text preview: Physics 41 Lecture 5 Lecture 5: Force; Newton’s First and Second Laws Goals of this Lecture ∙ Gain familiarity with using, and calculating, forces. Recall definitions from Lecture 1: ∙ Kinematics is the description of motion e.g. how the motion looks over time. ∙ Dynamics is the analysis of the causes of motions. So far we have only discussed kinematics, in particular how to determine the position and velocity of an object, given a particular acceleration. Now we move to dynamics, which will enable us to calculate the expected acceleration from a particular situation. 5.1 Force ∙ Accelerated motion of an object is caused by an interaction with the environment, and is described by a force . ∙ Force is a vector quantity, ⃗ u1D439 , that pushes or pulls an object in a particular direction. ∙ The units of force are Newtons where 1 Newton of force imparts an acceleration of 1m/s 2 to a 1 kg object. 5.1.1 Some Examples of Forces Some examples are now given of common forces in mechanics, together with the notation used both in the text and in these lectures notes. Note: The definitions of notation given below are not necessarily unique and may be different in different books. Always define your notation when solving problems. Gravitational Force: Massive objects such as the Earth exert a gravitational force on an object, ⃗ u1D439 G . The gravitational force exerted by the Earth on an object is referred to as weight and will be discussed in more detail in a future lecture. Spring Force: A compressed spring exerts a pushing force on an object, while a stretched spring exerts a pulling force. We will denote spring force by ⃗ u1D439 sp . Tension Force: A tension force is exerted by a string, rope or wire pulling on an object. Tension forces will be denoted by ⃗ u1D447 . Friction Forces: There are two kinds of friction force: Kinetic friction, ⃗ u1D453 u1D458 , opposes the motion of one object sliding over another; static friction, denoted ⃗ u1D453 u1D460 , is the force that keeps an object stuck in place on a surface and prevents its motion. Friction will be treated in detail in a later lecture. Thrust Forces: A jet or rocket engine expels fuel in one direction to cause motion in the opposite direction due to a force ⃗ u1D439 thrust . Normal Force: When you stand on the floor, or lean against a wall, the surface exerts a pushing force against you. This is a consequence of Newton’s third law (discussed next lecture) and will be denoted by ⃗u1D45B . Also, we will often write the force on a body u1D434 caused by a body u1D435 as ⃗ u1D439 B on A . 5.2 The Mass of an Object If a single force ⃗ u1D439 acts on a body of mass u1D45A , then it causes an acceleration ⃗u1D44E where: ⃗ u1D439 = u1D45A⃗u1D44E This is a consequence of Newton’s Second Law of Motion which we will discuss in detail in the next section. However, we note that we can assign a mass to a particular object by measuring the 1 Last updated January 10, 2010 Physics 41...
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This note was uploaded on 01/12/2010 for the course PHYSICS 41 taught by Professor Susskind,l during the Winter '08 term at Stanford.

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Lecture5 - Physics 41 Lecture 5 Lecture 5 Force Newton’s...

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