P1020_Newtons_Laws

P1020_Newtons_Laws - Physics 1020 Lecture 14 Newtons Laws...

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1 Physics 1020 – Lecture 14 Force and Mass (Ch. 5 Sect. 5-1) Newton’s First Law (Ch. 5 Sect. 5-2) Newton’s Second Law (Ch. 5 Sect. 5-3) Newton’s Third Law (Ch. 5 Sect. 5-4) Newton’s Laws of Motion (Walker Chapter. 5 )
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2 The Concept of Force (Sect. 5-1) Contact forces involve physical contact between two objects Field forces act through empty space * No physical contact is required Classes of Forces We will study dynamics (not a description of the motion but the causes of motion
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3 More About Forces A spring can be used to calibrate the magnitude of a force Forces are vectors , so you must use the rules for vector addition to find the net force acting on an object
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4 Force is a vector quantity! It matters not only how hard you push, but also in what direction object
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5 Vector Nature of Forces F 1 produces a 1 cm extension of the spring F 2 produces a 2 cm extension of the spring F 1 + F 2 produces a 3 cm extension of the spring
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6 Vector Nature of Forces When the forces are not aligned their vector nature must be considered: extension cm 24 . 2 2 1 2 2 2 2 2 1 = + = + = F F F 1 1 1 6 . 26 2 1 - atan atan 2 1 - = = - = F F θ
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7 Net force tension in string weight of block net force An object can have many forces acting on it at the same time. If all the forces oppose each other exactly then the net force = 0 What really matters is that when all the forces are added up that they don’t all cancel – net force
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8 Inertia and Mass The tendency of an object to resist any attempt to change its velocity is called inertia Mass is that property of an object that specifies how much resistance an object exhibits to changes in its velocity Mass (Sect. 5-1)
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9 More About Mass Mass is an inherent property of an object Mass is independent of the object’s surroundings Mass is independent of the method used to measure it Mass is a scalar quantity The SI unit of mass is kg
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10 Mass vs. Weight Mass and weight are two different quantities Weight is equal to the magnitude of the gravitational force exerted on the object * Weight will vary with location The mass of an object is the same everywhere Be careful, in everyday life we abuse these relations reporting our “weight” in kilograms
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11 (Newton’s First Law) If an object does not interact with other objects, it is possible to identify a reference frame in which the object has zero acceleration * This is also called the law of inertia * It defines a special set of reference frames called inertial frames , We call this an inertial frame of reference Inertial Frames
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12 Inertial Frames Any reference frame that moves with constant velocity relative to an inertial frame is itself an inertial frame A reference frame that moves with constant velocity relative to the distant stars is the best approximation of an inertial frame * We can consider the Earth to be such an inertial frame although it has a small centripetal acceleration associated with its motion
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P1020_Newtons_Laws - Physics 1020 Lecture 14 Newtons Laws...

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