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StaticEquilibrium

# StaticEquilibrium - SE-1 Static Equilibrium An object is in...

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SE-1 Static Equilibrium An object is in static equilibrium (it is not moving) IF 1) it is not translating (not moving up, down, left, or right) AND 2) it is not rotating (not spinning CW or CCW) (We are talking about motion in a 2D plane here.) If a stationary mass is acted on by several forces 1 2 3 F , F , F ,.. G G G , then in order to NOT translate, the net force must be zero. G G G net total 1 2 3 F F F F F F = = = + + + = 0 G G G 0 1x 2x 3x 1y 2y 3y F F F 0 , F F F + + + = + + + = x y F 0 , F 0 = = Equilibrium possible, but not guaranteed. Even though the net force is zero, the object might not be in static equilibrium. Here is a case (two forces acting on a bar) where the net force is zero, but the forces cause the object to spin : F In order to guarantee static equilibrium, we must have 1) net force = 0 AND 2) net torque = 0 F Remember what torque: torque (pronounced "tork") is a kind of "rotational force". r F τ Unit of torque = [ ] [ ][ ] r F m N τ = = magnitude of torque: ©University of Colorado at Boulder r = "lever arm" = distance from axis of rotation to point of application of force F = component of force perpendicular to lever arm vector Example: Wheel on a fixed axis: Notice that only the perpendicular component of the force will rotate the wheel. The component of the force parallel to the lever arm (F || ) has no effect on the rotation of the wheel.

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