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# W10.4 - Physics for Scientists Engineers 1 Lecture 37...

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March 20, 2008 Physics for Scientists&Engineers 1 1 Physics for Scientists & Physics for Scientists & Engineers 1 Engineers 1 Lecture 37

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March 20, 2008 Physics for Scientists&Engineers 1 2 Equilibrium Condition Equilibrium Condition Static Equilibrium Condition 1: We can use this 0 net force equilibrium condition to solve for unknown forces. Example: If object 1 rests on object 2, then the normal force , , keeps object 1 at rest and therefore the net force on that object 0. If would be larger, object 1 would have to lift off. If were smaller, it would sink into object 2. An object can only stay at rest, if the net force acting on it is exactly 0. ! F net = 0 ! N ! N ! N
March 20, 2008 Physics for Scientists&Engineers 1 3 Equilibrium Condition Equilibrium Condition In Cartesian coordinates, this vector equation is actually three independent equations: F net , x = F i , x i = 1 n ! = F 1, x + F 2, x + ... + F n , x = 0 F net , y = F i , y i = 1 n ! = F 1, y + F 2, y + ... + F n , y = 0 F net , z = F i , z i = 1 n ! = F 1, z + F 2, z + ... + F n , z = 0

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March 20, 2008 Physics for Scientists&Engineers 1 4 Center of Gravity Center of Gravity Where do we attach the gravity force vectors to the object in a free-body diagram? Center of Gravity Exists for every object Defined as the point where we can imagine the mass of an object to be concentrated Objects with uniform mass density: center of gravity is at the geometrical center
March 20, 2008 Physics for Scientists&Engineers 1 5 Note: Marked the center of notebook via intersecting lines! Center of Gravity Example Center of Gravity Example Again, holding the notebook computer Hand needs to support notebook below center of gravity - otherwise it will tip over! Center of gravity ! F g

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March 20, 2008 Physics for Scientists&Engineers 1 6 Center of Gravity Center of Gravity General Principle: If an object (or group of objects) is not attached to other objects, it can only be stable, if its center of gravity is supported from directly below (Object can also be suspended from a string directly above the Center of Gravity)
March 20, 2008 Physics for Scientists&Engineers 1 7 Net Torque Net Torque

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W10.4 - Physics for Scientists Engineers 1 Lecture 37...

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