1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Moment about a point = 0

  1. Oct 7, 2015 #1
    1. The problem statement, all variables and given/known data
    In this question , I'm asked to find the magnitude of P .

    2. Relevant equations


    3. The attempt at a solution
    Resultant moment about O =
    -10cos30 (4√3) -10sin 30 (10
    +15sin30(2) + 15cos30(4√3)
    -10cos30(4√3) -10sin30(4)
    -15cos30(4√3) -15sin30(7)
    +10cos30(4√3) +10sin30(11)
    +P cos 60 (4√3) =0
    P=22N , but the ans given is 15.6N , which part i did wrongly ?

     

    Attached Files:

  2. jcsd
  3. Oct 7, 2015 #2

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    What happened to ##P\,\sin(60^\circ)## ?
    Never mind, Py is zero.

    But what is ##4\sqrt 3## ?

    I also wonder what the 1, 2, 4, 7, 11 stand for.

    This time I would bet on the book answer (15.58846), even though they draw 30, 45 and 60 degree angles all alike...:smile:
     
    Last edited: Oct 7, 2015
  4. Oct 7, 2015 #3
    it's parallel with 4surd 3 , so , i didnt take in into calculation .
    tan30=4/A
    A=4 surd 3
     
  5. Oct 7, 2015 #4
    I suggest redrawing the sketch roughly to scale. Except for P, all the forces are on parallel lines. Extend those lines and draw a common perpendicular from them through O. All triangles are 30-60-90 and all distances can be gotten without trig using the 1:2:sqrt(3) ratios.
     
  6. Oct 10, 2015 #5
    you gt the ans = 15.5846 ? how do u gt it ? can you help ?
     
  7. Oct 10, 2015 #6

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Sure, that's what we are for.
    (1) I understand (##P_y=0##). But (2) ? Where do you need ##4/\tan(30^\circ)## ?

    My question on the 1,2,4,7, 11 was to set you thinking about what these stand for. They are not the x coordinates of the points where the forces act, if that's what you think....
     
    Last edited: Oct 11, 2015
  8. Oct 12, 2015 #7

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    For example, the leftmost 10 N clearly grabs to the left of O, so it should have a negative x-coordinate. Idem leftmost 15 N.

    Perhaps this also helps you in the other thread (the one on graph paper)
     
  9. Oct 13, 2015 #8
    i gt A = 4/tan30 due to it's a triangle . the horizontal length is 4m, the angle between the P and y-axis is 30 degree , then what does 1, 2, 4, 7, 11 stand for ?
     
  10. Oct 13, 2015 #9
    If i am to take the 1, 2, 4, 7, 11 ans the r , then is my working correct ? ( ignore the ans given ) , is my concept correct ?
     
  11. Oct 13, 2015 #10

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    In reply to your post #9:

    r has two coordinates, so: No.

    In a cartesian coordinate system with x horizontal and y vertical and O at the origin, you want the x coordinate and the y coordinate of the point where the leftmost 10 N acts. etcetera.

    ----

    In reply to your post #8: I asked you first :smile: ! (in post #2)

    From your post #1:

    Resultant moment about O =
    -10cos30 (4√3) -10sin 30 (10 ##\qquad ## I suppose you mean (1)
    +15sin30(2) + 15cos30(4√3)
    -10cos30(4√3) -10sin30(4)
    -15cos30(4√3) -15sin30(7)
    +10cos30(4√3) +10sin30(11)
    +P cos 60 (4√3) =0​

    The red numbers 1, 2, 4, 7, 11 are wrong. So are all the 4√3

    I can't find any A in your picture. What do you mean with A ?
    (I don't mean 4/tan30, I mean where is it shown in your picture -- which please redraw to scale as insightful also suggested! -- it avoids more than half of your errors !)
    The angle between P and the y-axis is clearly 90##^\circ##

    ----
     
  12. Oct 13, 2015 #11
    A is the distance of O to horizontal line where it join all the 15N and 10N
     
  13. Oct 13, 2015 #12

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Well, you have the wrong value.
     
  14. Oct 13, 2015 #13
    Then , wat is the correct one ?
     
  15. Oct 14, 2015 #14

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    For me to know and for you to find out (sorry, that's the PF culture: the answer doesn't help you, you only learn by discovering for yourself). A little hint: it is considerably more than ##4\sqrt 3##. Did you make a drawing ?
     
  16. Oct 14, 2015 #15

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    recognize this picture ? If ##|\vec A| = 12 ##, what is ##A_x## and what is ##A_y## ?
    upload_2015-10-14_12-5-46.png
     
  17. Oct 14, 2015 #16
    Ax = 12cos60 , Ay = 12 sin 60
     
  18. Oct 14, 2015 #17

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    So Ax = 6 and Ay is ... (not ##4\sqrt 3##).

    Proceed to list the x-coordinates for the various points where the 10N, 15, 10, 15, 10 N act.
     
  19. Oct 14, 2015 #18
    ok , then I'm sure the 1,2,4,7,11 is not correct ? what does those numbers mean ? then , how to determine the perpendicular distance for other forces which is situated at the right of the first 15N and 10N ?
     
  20. Oct 14, 2015 #19

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Well, you have the y coordinates (at least if you filled in the dots in post #17 -- what did you get ?) and you have the horizontal distances to the leftmost point of the horizontal line where all these 10, 15 N forces act. What is the x coordinate of this leftmost point ?

    I figured your post #1 doesn't deal with perpendicular distances but instead evaluates the ##\ \vec\tau = \vec r \times \vec F \ ## expression (##\ \tau_z = r_x F_y - r_y F_x \ ## )​
     
    Last edited: Oct 15, 2015
  21. Oct 14, 2015 #20
    i take the x coordinate of this leftmost point as {0, 0)
    isnt the r in the expression (##\ \tau_z = r_x F_y - r_y F_x \ ## ) represent perpendicular distance ? why you said my r is not perpendicular distance ?
    the y coordinates are all 12sin60= 6 surd(3)
     
    Last edited: Oct 14, 2015
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Moment about a point = 0
  1. Moment about a point (Replies: 1)

  2. Moment about a point (Replies: 6)

Loading...