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# W4.1 - Physics for Scientists Engineers 1 Spring Semester...

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January 25, 2008 Physics for Scientists&Engineers 1 1 Physics for Scientists & Physics for Scientists & Engineers 1 Engineers 1 Spring Semester 2008 Lecture 12

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January 25, 2008 Physics for Scientists&Engineers 1 2 Force Force So far: describe motion of objects! Kinematics Now: find out what makes objects move! Dynamics Concept of a force When objects interact with each other, they exert force(s) on each other Different types of interaction: different forces Contact forces: normal force, friction force Action-at-a-distance forces: gravity, electromagnetism
January 25, 2008 Physics for Scientists&Engineers 1 3 Weight and Mass Weight and Mass Magnitude of gravitational force on an object = weight Gravitational force on an object, F g , is always proportional to its mass Near surface of Earth (altitude of 10 km or less): Weight is constant and is the product of object’s mass and the Earth’s gravitational acceleration: F g = mg g = 9.81 m/s 2

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January 25, 2008 Physics for Scientists&Engineers 1 4 Example: Object with mass m = 5.00 kg Gravitational force F g = mg = (5.00 kg)(9.81 m/s 2 ) = 49.05 kg m/s 2 Force Unit 1 kg m/s 2 = 1 N Named after British physicist Sir Isaac Newton (1642-1727), father of modern mechanics and perhaps the most influential scientist who ever lived Summary: mass, m , is measured in units of kg whereas weight (a force!), mg , is measured in units of N Weight and Mass Weight and Mass
January 25, 2008 Physics for Scientists&Engineers 1 5 Example: Gravitational Force Example: Gravitational Force on Notebook Computer on Notebook Computer Hold computer Feels heavy, because gravity is pulling on it Gravitational force has direction: straight down Introduce standard Cartesian coordinate system ! F g = ! mg ˆ y

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January 25, 2008 Physics for Scientists&Engineers 1 6 Net Force Net Force Definition: Net force = vector sum of all forces acting on a given object
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