Before you go to the hardware store you need to calculate what should be the

# Before you go to the hardware store you need to

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Before you go to the hardware store, you need to calculate what should be the minimum tensile strength of the cable. 1 m B C 1 m 45 ° 1. system = just the sign 2. objects in its surroundings that it interacts significantly with: B,C, Earth x y z +z out of page

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Static Equilibrium of a Hanging a Sign 3. applying the momentum principle to the system produces 2 non-trivial equations (x,y directions) for 3 unknowns: F C , F B , and direction of F B . We need another to solve. 4. applying ang. mom. principle provides another independent equation x y z +z out of page 45 ° C F arrowrightnosp 1 m 1 m B F arrowrightnosp Earth F arrowrightnosp CM , , A sys A net dL dt τ = arrowrightnosp arrowrightnosp , , 0 0 A sys A net dL dt τ = = arrowrightnosp arrowrightnosp What’s a “good” choice for picking point A? The place where the bar touches the sign, since the unknown F B exerts no torque about this point and hence doesn’t show up in the equation! A static situation
Q1. 1 m 1 m B F arrowrightnosp Earth F arrowrightnosp CM What is the torque about point A exerted on the sign by the Earth? A x y z +z out of page mg A. 0, , 0 2 B. 0, , 0 2 mg C. 0, 0, 2 D. 0, 0, 2 E. 0, 0, 0 N m mg N m N m mg N m N m - - 45 ° C F arrowrightnosp

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Q2. 1 m 1 m B F arrowrightnosp Earth F arrowrightnosp CM What is the torque about point A exerted on the sign by the cable? A x y z +z out of page C C C C C C C C A. 0, 0, F 2 B. F cos45 , F sin45 , 0 C. 0, 0, F cos45 + F sin45 D. 0, 0, F cos45 F sin45 E. 0, 0, F N m N m N m N m N m ° ° ° ° - °- ° 45 ° C F arrowrightnosp
Static Equilibrium of a Hanging a Sign 5. Ang. Mom. Principle applied to our system consisting of just the sign. x y z +z out of page 45 ° C F arrowrightnosp 1 m 1 m B F arrowrightnosp Earth F arrowrightnosp CM , , , 0 A net A Earth A C τ τ τ = + = arrowrightnosp arrowrightnosp arrowrightnosp You need a cable that can support 173 N at a bare minimum .

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