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Statics question

  1. Oct 11, 2006 #1
    I'm really stuck on how to start these two questions. Could someone please give me some hints? Any help would be really appreciated.

    1. A body of mass 1.5kg is placed on a plane surface inclined at 30 degrees to the horizontal. Calculate the friction and the nornal reaction which the plane must exert if the body is to remain at rest.

    2. A rectangular picture, of mass 9.1kg, is suspended from a hook by two equal strings attached to its upper corners. If the tension in both strings is 49N, what is the angle between the strings?

    Thank you.


    P.S. - I've tried to draw diagrams but I don't think that they are right.
  2. jcsd
  3. Oct 11, 2006 #2


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    Staff: Mentor

    -1- When you draw the free body diagram, how many forces do you get? Which ways do those force vectors point?

    -2- If the picture were held by two vertical strings, what would be the tension in each string? What is different if the two strings are angled inward to the same point?
  4. Oct 11, 2006 #3
    I've really got no idea how to draw the free body diagram.

    If the two strings are angled inwards to the same point, would the resultant horizontal force be zero?
  5. Oct 11, 2006 #4
    Thanks for replying by the way!
  6. Oct 11, 2006 #5


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    Staff: Mentor

    Here's a pointer to free body diagram info at wikipedia.com (but the figure isn't serving up right unfortunately):


    -2- You are correct that the resultant horizontal force is zero, and the resultant vertical force is the same as if the strings were vertical. But angling the strings in causes an increase in the tension in each one -- why? and how much?
  7. Oct 11, 2006 #6
    Has anyone please got any idea how to solve this question?

    4. A mass of 40kg is suspended by two ropes inclined to the vertical at angles of 30º and 80º.

    (a) Call the tensions T30 and T80, and write down the vertical component of each tension.

    (b) By considering the size of the resultant vertical force, write down an equation linking the tensions with the weight of the mass.

    (c) By considering the size of the resultant horizontal force, write down an equation linking the tensions together.

    (d) Using the previous two answers, find T30 and T80.

    Thank you.

  8. Oct 11, 2006 #7
    Also, I don't really get this one either. I'll attach a (very) rough diagram that I've copied from the sheet.

    1. Calculate:

    (a) the tension in the strings A and B (trivial),

    (b) the tension in the string C,

    (c) the mass m.

    For part c I know that W = mg and that g = 9.81, but how do I work out W?



    Attached Files:

  9. Oct 11, 2006 #8
    have you drawen the free body with forces?
  10. Oct 11, 2006 #9
    No, because I really don't get how to draw the diagram.
  11. Oct 11, 2006 #10
    ok it says suspended so? hanging from right? You have one mass held up by 2 ropes. those to ropes are not hung straight up, but at angles to the vertical axis. so the ropes form some what of a "V." draw a straight line through the middle of those 2 ropes (through the "v"). where the ropes com together at the bottom with your straight line "y axis" in the middle create an angle of 30 deg. to one side and 80 deg. to the other side. It would look very similar to the drawing you posted except get rid of the 2 additional masses and ropes on the sides. Does this help???
  12. Oct 11, 2006 #11
    Thanks so much! That has really helped! :smile:

  13. Oct 11, 2006 #12
    So for T30 I got the vertical componet to be 34.6 N. Is this correct? Also, is the vertical component the same for T80?

    Last edited: Oct 11, 2006
  14. Oct 11, 2006 #13
    what is the work you have done? What resultant equations do you have?
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