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Chapter_5

# Chapter_5 - Chapter 5 Force and Motion(5-1 Chapters 2 and 4...

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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 Newton’s three laws of motion which are at the heart of classical mechanics. Newton’s laws describe physical phenomena of a vast range, including the motion of objects from proteins and cells to stars and planets. Newton’s 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 = 3×10 8 m/s). In this case we must use Einstein’s 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)

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Newton’s 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 : “ Newton’s first law If no force acts on a body, the body’s velocity cannot change (i.e. the body’s 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

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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?

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Relationship between F, m and a 2 2 at L a 2L / t 2 = = m Given F, m, measure t 0 a 0 F m Given 2F, m, measure t 1 a 1 /a 0 =(t 0 /t 1 ) 2 =2 a F 2F 2m Given F, 2m, measure t 2 a 2 /a 0 =(t 0 /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 θ φ θ φ r r r r r r r r 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: We place an object of mass m = 1 kg on a frictionless surface and measure the acceleration a that results from the application of a force F .

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Chapter_5 - Chapter 5 Force and Motion(5-1 Chapters 2 and 4...

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