Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Resultant force and angles please help.

  1. Feb 25, 2012 #1
    1. The problem statement, all variables and given/known data

    A spider is resting after starting to spin its web. The gravitational force on the spider is 0.165 N on the junction of three strands of silk. The junction is supported by different tension forces in the two strands above it so that the resultant force on the junction is zero. The two sloping strands are perpendicular (the 2 strands above it), and we have chosen the x and y directions to be along them. The tension Tx is 0.112 N.

    (a) Find the tension Ty.


    (b) Find the angle the x axis makes with the horizontal.


    (c) Find the angle the y axis makes with the horizontal.

    My attempt at this:
    For the first one I did
    so x=0.053
    which was wrong (because I thought the resultant force =0)
    I think the second and third depend on the first.
    but if I get the first maybe my answer to the second and third may be
    cos^-1(y(the first one)/the magnitude)?
    I am not sure where to start really, can someone point me in the right direction?
  2. jcsd
  3. Feb 25, 2012 #2
    If I understand the problem correctly, the spider sits on the vertical leg so it looks like the letter 'y' with the angle of the vee being 90 degrees.

    If that is correct set up a force balance at the point where all 3 pieces come together. The vertical forces must sum to the weight of the spider and the horizontal forces must balance.

    Looking at the equations, you will have 3 equations with 3 unknowns.

    Hint: sin(alpha)=cos(90-alpha), etc
  4. Feb 25, 2012 #3
    Thanks for replying...
    unfortunately I didn't undersand anything from what you just said,
    can you expand?
    annd yes the V is 90degrees
  5. Feb 25, 2012 #4
    Do you know what a force balance is? Do I have the geometry correct?
  6. Feb 25, 2012 #5
    No I don't
    Would be great if you could explain :D
  7. Feb 25, 2012 #6
    Since nothing is moving in this problem, all forces must balance out. They must sum to zero so sum them in the vertical and horizontal directions. Write equations at the vee that represent this. When you do that you will wind up with three equations and 3 unknowns that you'll have to solve to determine the tension in the other leg of the vee.
  8. Feb 25, 2012 #7
    how do I sum them in the vertical and horizontal direction?
  9. Feb 25, 2012 #8
    You would use trigonometry.
  10. Feb 25, 2012 #9
    can you be more specific?
  11. Feb 25, 2012 #10
    You do not sound serious to me. You'll have to find someone else to help.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook