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Chapter5 - Chapter 5 Demos...

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PHYS276, S09 Chapter 5 1 Chapter 5 Demos: http://www.physics.umd.edu/deptinfo/facilities/lecdem/lecdem.htm A stiff spring (that you can compress by hand) B2-03: equilibrium of forces, inclined plane C6-11: sliding friction – lecture table and felt C4-21: atwood machine C5-01: static dynamometers Thanks to Prof LaPorta and “Peer Instruction” by Eric Mazur
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EXAM 1 is on Wednesday!!! PHYS276, S09 Chapter 5 2
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Chapter 5 Consider the Atwood Machine m 2 m 1 Two masses are connected by a string that passes over a light, frictionless pulley. Describe the motion of one of the masses. C4-21: atwood machine
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PHYS276, S09 Chapter 5 4 What are Force and mass? See your text for a “real” definition. However, for the purposes of this semester. Force : A push or pull. mks unit: Newton. A vector. For this semester, all forces will be CONTACT forces, except for gravity. Therefore, except for gravity, to look for possible forces that act ON an object, consider what is touching an object. (next semester you’ll learn about other “field” forces such as electricity, magnetism, the strong force, etc) Mass : an intrinsic property of an object that determines its response to a force. mks unit is kg. Mass is a scalar. Note that mass and weight are not the same thing. Weight is a force (and therefore a vector) due to gravity.
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Forces and Objects PHYS276, S09 Chapter 5 5 Define an object or “system”. Define external and internal forces.
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PHYS276, S09 Chapter 5 6 Types of Force Some specific forces we will be considering this semester. • gravitational force • “normal” force • friction • tension in a rope • drag forces (like wind resistance) • force from a compressed spring
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PHYS276, S09 Chapter 5 7 Normal Force Normal Force: the surface of a rigid solid acts a bit like a very stiff spring. It deforms at a microscopic level and produces enough upward force on the object to counteract the other forces on it and stop the motion perpendicular to the objects surface. The size of this force is variable. It adjusts itself to stop the motion in the direction perpendicular to the surface. “normal” means perpendicular! Be careful! If there are more than 1 object in your drawing, be clear in your mind which “object” you are doing force analysis to. Demonstrate with spring
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PHYS276, S09 Chapter 5 8 Newton’s 3 Laws • if the (vector, obviously) sum of the external forces (“Net Force”) on an object is zero, the object will move at constant velocity • When there is a net external force on an object, that object will have an acceleration • If one objects exerts a force on a second object, the second objects exerts an equal and opposite force on the first. (and thus the net of an internal force is zero) Very simple to state, applicable to a wide variety of problems, but requires careful thought to correctly apply.
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