Chapter_5 - Chapter 5 Force and Motion Chapters 2 and 4:...

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Unformatted text preview: Chapter 5 Force and Motion Chapters 2 and 4: studied kinematics to describe the motion of objects without knowing what cause the motion. Chapters 5 and 6: will study dynamics . In this chapter we will introduce Newtons three laws of motion which are at the heart of classical mechanics. Newtons laws describe physical phenomena of a vast range, including the motion of objects from proteins and cells to stars and planets. Newtons laws fail in the following two circumstances: 1. When the speed of objects is near (1% or more) the speed of light in vacuum (c = 310 8 m/s). In this case we must use Einsteins special theory of relativity (1905) 2. When the objects are very small (e.g. electrons, atoms etc). In this case we must use quantum mechanics (1926) (5-1) Newtons First Law Scientists before Newton thought that a force was required to keep an object moving at constant velocity. Right? Wrong! This mistake was made before friction was recognized to be a force. For example, an object sliding on a slippery floor Newton proposed the correct form of what is since known as : Newtons first law If no force acts on a body, the bodys velocity cannot change (i.e. the bodys acceleration is zero) (5-2) 5.3.1. A pipe is bent into the shape shown and oriented so that it is sitting horizontally on a table top. You are looking at the pipe from above. The interior of the pipe is smooth. A marble is shot into one end and exits the other end. Which one of the paths shown in the drawing will the marble follow when it leaves the pipe? a) 1 b) 2 c) 3 d) 4 e) 5 Practice question 5.3.1. A pipe is bent into the shape shown and oriented so that it is sitting horizontally on a table top. You are looking at the pipe from above. The interior of the pipe is smooth. A marble is shot into one end and exits the other end. Which one of the paths shown in the drawing will the marble follow when it leaves the pipe? a) 1 b) 2 c) 3 d) 4 e) 5 Force: tentatively defined as a push or pull action exerted on an object. In physics, we define a force exerted on an object by measuring its effect --- the acceleration it causes How does force determine acceleration? Relationship between F, m and a 2 2 at L a 2L / t 2 = = m Given F, m, measure t a F m Given 2F, m, measure t 1 a 1 /a =(t /t 1 ) 2 =2 a F 2F 2m Given F, 2m, measure t 2 a 2 /a 0 =(t /t 2 ) 2 = 0.5 F a 1/ m F=ma L , , If several forces act on a body ( , , and ), the net force is defined as: cos( ) cos( ) s ( ) s ( ) = + + = - + = +- A B C ne net A B C net x A C net y B t A C F F F F F F F F F F F i F F n F in F r r r r r r r r Force: tentatively defined as a push or pull action exerted on an object....
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Chapter_5 - Chapter 5 Force and Motion Chapters 2 and 4:...

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