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Homework Help: Theoretical question about forces

  1. Jul 3, 2010 #1
    1. The problem statement, all variables and given/known data
    The mayor of a city decides to fire some city employees because they will not remove the sag from cables that support the city traffic lights. If you were a lawyer and knew some physics, what defence would you give on behalf of the employees? Who do you think will win the case in court?

    2. Relevant equations

    3. The attempt at a solution
    I'm thinking of the theory of Forces in equilibrium. Even if you remove the sag by increasing the tension in the cables, the light will always be motionless, but there's always the force of gravity pulling it down. Thus, the cables will still sag.

    Does this make sense?
  2. jcsd
  3. Jul 3, 2010 #2


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    It sounds like you have the right idea. You could also draw a free-body diagram for the traffic light, showing the forces from gravity and cable tension.
  4. Jul 3, 2010 #3
    It does make sense that the cable will still sag, but if you increase the tension, it will sag less. What will happen if the cable is pulled tighter and tighter, thus raising the light and shortening the cable? This problem is related to the classic physics problem where you show that it takes infinite tension to make the cable perfectly straight with no sag.

    Now ask, "Why would an engineer design the sag into the system?".

    It seems the Mayor wants to save money by using less cable, but he clearly does not understand physics and engineering. The engineer will design based on keeping the light at an acceptable height and minimizing the cable cost. How is cost of the cable related to it's length? How is the cable cost related to it's thickness?
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