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Unformatted text preview: Physics 41 Lecture 5 Lecture 5: Force; Newtons 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 oor, or lean against a wall, the surface exerts a pushing force against you. This is a consequence of Newtons 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 = u1D45Au1D44E This is a consequence of Newtons 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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