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# chapter4.5_slide - Important step draw free-body diagram...

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9/22/2009 1 Important step: draw free-body diagram Example: Pulling a crate How many forces acting on the crate? (Neglecting the friction) Equation of motion of crate: x-component: y-component: No acceleration along y g m F g g n F F T n F g F T ma x 0 mg F ma n y Objects that are either at rest of moving with a constant velocity are said to be in equilibrium In other words, the vector sum of the external forces on a object in equilibrium is zero. 0 net F Equilibrium 0 a 0 0 : 2D , , y net x net F F

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9/22/2009 2 Traffic light weigh: 100 N Find the tension in each of the three cables Free-body diagram of traffic light In the y-direction: Free-body diagram of the knot 0 3 g F T x y N 100 3 mg F T g ' 3 T Reaction force to 3 T 3 ' 3 T T Equilibrium condition of knot - in the x-direction - in the y-direction Two equations, two variables. Solve Eq (1) for T
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chapter4.5_slide - Important step draw free-body diagram...

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