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Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova Sections 807, 808, 809 Lecture 8
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Quiz (folders)
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Reading quizzes (Chs 5,6)
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Hockey Puck Which of these three best represents a hockey puck in the real world? a) b) c)
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Newton’s 1 st Law A body acted on by no net force moves with constant velocity (which may be zero) and zero acceleration
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Aristotle: a natural state of an object is at rest; a force is necessary to keep an object in motion. It follows from common sense. Galileo: was able to identify a hidden force of friction behind common- sense experiments 1564-1642 384-322 B.C.
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Galileo: If no force is applied to a moving object, it will continue to move with constant speed in a straight line Galilean principle of relativity: Laws of physics (and everything in the Universe) look the same for all observers who move with a constant velocity with respect to each other. Inertial reference frames
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Newton’s second law The vector acceleration of an object is in the same direction as the vector force applied to the object and the magnitudes are related by a constant called the mass of the object.
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A Recipe for Solving Problems 1.Sketch Isolate the body (only external forces but not forces that one part of the object exert on another part) 2. Write down 2 nd Newton’s law Choose a coordinate system Write 2 nd Newton’s law in component form: 3. Solve for acceleration
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Is it better to push or pull a sled? You can pull or push a sled with the same force magnitude, F P, and angle , as shown in the figures. Assuming the sled doesn’t leave the ground and has a constant coefficient of friction, , which is better? FPFP FPFP
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Kinetic Friction For kinetic friction, it turns out that the larger the Normal Force the larger the friction. We can write F Friction = Kinetic N Here is a constant Warning: –THIS IS NOT A VECTOR EQUATION!
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Static Friction This is more complicated For static friction, the friction force can vary F Friction Static N Example of the refrigerator: –If I don’t push, what is the static friction force? –What if I push a little?
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H Coefficient of friction: What is the normal force? What is the velocity of the block when it reaches the bottom?
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Newton’s 3 rd Law For every action there is an equal, but opposite, reaction
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Skater Skater pushes on a wall The wall pushes back –Equal and opposite force The push from the wall is a force –Force provides an acceleration –She flies off with some non-zero speed
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P m1m1 m2m2 No friction m1m1 Free body diagram m1m1 m1gm1g P N1N1 F 21 m2gm2g F 12 N2N2 m2m2 F 12 =F 21
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Quiz a) A crate of mass m is on the flat bed of a pick up truck. The coefficient of friction between the crate and the truck is . The truck is traveling at the constant velocity of magnitude V 1. Draw the free body diagram for the crate. b) The truck starts to accelerate with an acceleration a c. Draw the free body diagram for the crate, if the crate does not slip.
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A small block, mass 2kg, rests on top of a larger block, mass 20 kg. The coefficient of friction between the blocks is 0.25. If the larger block is on a frictionless table, what is the largest horizontal force that can be applied to it without the small block slipping?
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Have a great day! Reading: Chapter 5,6 Hw: Chapter 6 problems and exercises
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