Problem Description
Click to view movie (98k)

Susan, a new engineering student, is being very careful to write down each calculation step as she proceeds through a complex problem. Her friend Sam, on the other hand, wants to round all numbers to only two digits to get done faster.

What is known:

  • The traffic light weighs 146.7 lb.
  • Cable A is 17.49° from the horizontal.
  • Cable B is 11.46° from the horizontal.

What is Statics?
Click to view movie (99k)


What is the tension in both cables using four significant digits and two significant digits? Does rounding numbers to only two digits still provide an accurate answer?


Force Diagram
  • Solve the problem using the basic steps in engineering problem solving: 1) read 2) draw diagrams 3) write equations 4) solve and 5) check.
  • For step 3, use the summation of forces in both the x and y directions:

                      ΣFx = 0        ΣFy = 0
  • Solve twice, once with four significant digits and once with two significant digits.