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Homework Help: Mechanics tension in string

  1. Mar 18, 2010 #1
    This isn't really homework, just doing some revision.

    1. The problem statement, all variables and given/known data
    Find the tension in each string. Two spheres both of mass, m = 15kg. Diameter of each is 25cm. Each string 35cm long. Spheres are suspended on strings with other end attached to same point on ceiling. Therefore spheres are touching. Hopefully, that is not too difficult to understand, sorry no picture!

    Taking g as 10 m/s/s

    2. Relevant equations plus attempt
    A vertical line drawn from point where string attached to ceiling makes a triangle with string + radius (hypotenuse) and radius of ball (opposite side).
    hypotenuse = 0.35 + 0.125 = 0.475 meters
    therefore angle alpha = arcsin(opp/ hyp) = arcsin(0.125 / 0.475)

    Tension in each secondary string = T2 = mgcos(alpha) = 145 Newtons

    However answer at back of book gives 152N - how can this be? g cannot be greater than 10! Really would like some help
  2. jcsd
  3. Mar 18, 2010 #2


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    How many forces act on each sphere?
  4. Mar 19, 2010 #3
    well, i'd say for each sphere gravity (mg), which pulls directly downward, causing a tension in the string, which acts along string at an angle alpha from the vertical. also the spheres are in contact, so there is a contact force in the x-direction.

    still i dont understand. if g = 10 m/s/s, then weight of each sphere is 150N. how can the tension in each supporting string be greater than weight of the thing it supports. I would love an example.
  5. Mar 19, 2010 #4


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    You have an example. Draw a free body diagram for one of the spheres and put in all the forces. In what direction is the contact force? Say that the sum of all vertical components is zero, solve for the tension and see what you get.
  6. Mar 19, 2010 #5
    thank you!

    sum Fy = 0 = Tsin(a) - mg >>> Tsin(a) = mg

    sum Fx = mgsin(a)cos(a)

    T = sqrt(Fx^2 + Fy^2) = 152 , when g = 9.8

    is that what you were hinting at?
  7. Mar 19, 2010 #6


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    Not exactly. Look at your first equation. It says

    Tsin(a) = mg

    Can you solve it to find T?
    Given that the angle is not zero, is T greater than, equal to or less than mg?
  8. Mar 20, 2010 #7
    T >mg but I've worked the angle, a, to be 15.3 deg.

    T = mg/ sin(a) = 15*10 / sin(15.3) = 568N

    this is far too large.
    if length of string = 0.35m and radius of sphere = 0.125m, we have a triangle of:
    opposite side = radius = 0.125m
    hypotenuse = string + radius = 0.475m
    therefore sin(a) = opp/hyp = 0.125/ 0.475
    and angle a = arcsin(0.125/ 0.475) = 15.3 deg

    but i do see how T can be greater than mg now (at least mathematically), otherwise it still seems like a rather weird idea!
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