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Unformatted text preview: The Laws of Motion Chapter 4 • Describes the relationship between the motion of objects in our everyday world and the forces acting on them • Conditions when Classical Mechanics does not apply • very tiny objects (< atomic sizes) • objects moving near the speed of light Classical Mechanics • Usually think of a force as a push or pull • Vector quantity • May be a contact force or a Feld force • Contact forces result from physical contact between two objects • ¡ield forces act between disconnected objects • Also called “action at a distance” ¡orces Contact and Field Forces • An object moves with a velocity that is constant in magnitude and direction, unless acted on by a nonzero net force • The net force is deFned as the vector sum of all the external forces exerted on the object Newton’s ¡irst Law • External force • Any force that results from the interaction between the object and its environment • Internal forces • Forces that originate within the object itself • They cannot change the object’s velocity External and Internal Forces • Is the tendency of an object to continue in its original motion Inertia • A measure of the resistance of an object to changes in its motion due to a force • Scalar quantity • SI units are kg Mass • The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. • F and a are both vectors • F and a point in the same direction • Can also be applied threedimensionally Newton’s Second Law ~ a = ⌃ ~ F m ⌃ ~ F = m ~ a • SI unit of force is a Newton (N) • US Customary unit of force is a pound (lb) • 1 N = 0.225 lb • See table 4.1 Units of Force • 1642 – 1727 • Formulated basic concepts and laws of mechanics • Universal Gravitation • Calculus • Light and optics Sir Isaac Newton Net Force Example 1 • Vector sum of forces • What is the net force acting on a book sitting motionless on the table? • Show the net force by drawing all forces acting on the book. ~ F n ~ F g ~ a = 0 ~ F net = ma ~ F net = 0 Net Force Example 2 • Apply horizontal force to book and draw new free body diagram. (assume no friction for now) • Show resultant force. • Does the book accelerate? • What direction? ~ F n ~ F g ~ a =? ~ F Net Force Example 3 • Now apply vertical force less than F g and draw free body diagram (assume no friction for now). • Show resultant force. • Does the book accelerate? • Does the normal force change?...
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This note was uploaded on 11/09/2011 for the course PHYS 1410 taught by Professor Staff during the Fall '08 term at Texas State.
 Fall '08
 staff
 Force

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