physic chapter 5 outline

physic chapter 5 outline - Chapter 5 The Laws of Motion Sir...

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Chapter 5 The Laws of Motion

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Sir Isaac Newton 1642 – 1727 Formulated basic laws of mechanics Discovered Law of Universal Gravitation Invented form of calculus Many observations dealing with light and optics
Classes of Forces Contact forces involve physical contact between two objects Examples a, b, c Field forces ( noncontact forces) act through empty space No physical contact is required Examples d, e, f

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Non-contact force: gravitational force The gravitational force, , is the force that the earth exerts on an object This force is directed toward the center of the earth g F r
Contact Forces: Examples 1. Normal force ñ is the force that the surface exerts on the object. The normal force is always perpendicular to the surface. 3. Tension force is the force that the rope exerts on the object. Acts always along the rope. T r

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Vector Nature of Forces You apply two forces: downward force F 1 and horizontal force F 2 . The resultant (or net) force is F 1 + F 2 Forces are vectors, so you must use the rules for vector addition to find the net force acting on an object
Newton’s First Law In the absence of external forces, an object at rest remains at rest and an object in motion continues its motion with a constant velocity Newton’s First Law tells us that when no force acts on an object or net force of all forces acting on the object is zero, the acceleration of the object is zero too.

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Newton’s Second Law In the presence of external force the object accelerates. The acceleration of an object is directly proportional to the force acting on it and inversely proportional to its mass Force is the cause of change in motion, as measured by the acceleration If there are more forces acting on the object, the acceleration is proportional to net force acting on it. Algebraically, m m = F a F a r r r r
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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More About Newton’s Second Law is the net force This is the vector sum of all the forces acting on the object Newton’s Second Law can be expressed in terms of components: Σ F x = m a x Σ F y = m a y Σ F z = m a z From Newton’s Second Law gravitational force is expressed as Its magnitude is called the weight of the object Weight = F g = mg F r g m = F g r r
Units of Force The SI unit of force is the newton (N) 1 N = 1 kg · m / s 2 The US Customary unit of force is a pound (lb) 1 lb = 1 slug · ft / s 2 1 N = 0.225 lb

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More About Weight Because it is dependent on g , the weight varies with location g , and therefore the weight, is less at higher altitudes This can be extended to other planets, but the value of g varies from planet to planet, so the object’s weight will vary from planet to planet Weight is not an inherent property of the object
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