Problem Diagram


Jim Knowitall just started a new job washing windows. He decides to spread the two ropes further apart at the top so the platform will be more stable (or so he thinks).

What is known:

  • A wind gust causes the platform to swing.
  • The ropes rotate at a constant angular velocity of 10°/s counterclockwise.
  • The current vertical distance is 20 feet.
  • The ropes are 30° off the vertical.
  • The platform is initially horizontal and is 6 feet wide.


What is the angular velocity of the platform when the ropes are spread at 30° to the vertical? Is it safe?


  • One approach is to use the relative velocity equation, as was done in the previous section.
  • A second, and sometimes easier approach is to use the concept of "Instantaneous Center of Zero Velocity" (IC).
  • To use the IC method, the velocities of any two points on the body are needed.
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