1. Not finding help here? Sign up for a free 30min 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!

Help ! can you please explain this to me ! thanks ! (it is all about vector )

  1. Jun 22, 2011 #1
    this problem was discussed by our professor . but im a lil bit confused on it ! thanks ! help !

    FIVE COPLANAR FORCES ACT ON AN OBJECT AND THEIR RESULTANT !

    our prof use this formula !

    Bx=Bsin(degree)
    By=Bcos(degree)

    ANSER:

    VECTORS X-COMPONENTS Y-COMPONENTS
    A 19.0 0
    B 7.5 12.99
    C -11.31 11.31
    D -9.53 -5.5
    E 0 -22.0
    Σx=5.66 Σy=-3.2

    MY QUESTION IS : HOW TO KNOW IF WHAT PARTICULAR OPERATION TO BE USE ON EACH OF THE COMPONENTS ? WHEN WILL I USE THE SIN FUNCTION AND COSINE FUNCTION IN GETTING THE COMPONENTS? THANKS !
     

    Attached Files:

    • A.jpg
      A.jpg
      File size:
      10.3 KB
      Views:
      71
  2. jcsd
  3. Jun 22, 2011 #2

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Have you studied trigonometry yet?
     
  4. Jun 22, 2011 #3
    yes. a year a go. i know how to use the function but my problem is i dont know when to use the cos and sin function like in the x-component my prof use the sin function in computing the x component except on letter D and C . can you please help me how to analyze it . i'm confuse !
     
  5. Jun 22, 2011 #4
    can you please elaborate it ? i cant understand :((
     
  6. Jun 22, 2011 #5

    rock.freak667

    User Avatar
    Homework Helper

    When in doubt just remember, the side opposite to the angle will have sine and the side adjacent will be the cosine.
     
  7. Jun 22, 2011 #6
    more more :(( please ! Im confuse !
     
  8. Jun 22, 2011 #7
    Okay,

    Lets put it differently. We need to find the resultant force on the body. In fact we need the sum of the forces. As forces are vectors, their direction also needs to be considered in the addition.

    Had all the forces been in the same direction, it would have been easier as it would simply be the sum of those.

    So what we do is, we choose two directions, preferably perpendicular, which by convention is chosen as the x-axis and y-axis.

    We resolve all these forces into these axes. Are you with me?

    How we do is like this... if F is the force then F.cos(t) will be the component in x-axis and F.sin(t) in the y axis, if t is the angle made by the force with x-axis.

    So we resolve all the forces like this. Sum up all the Fx's (aka force in x-direction) and Fy's

    then the resultant force will be ((sum of all Fx)^2 + (sum of all Fy)^2)^0.5

    Please let me know if you still have any doubts.
     
  9. Jun 22, 2011 #8
    According to your figure, my x-axis would be your A and my y-axis would be your negative-E direction.

    And find the angle that each force each force is inclined with A in the counter clockwise direction from A. For example B makes 60' with A, C makes 135', D makes 210' and E makes 270' with A. So that is the 't' for each force in my equation.
     
  10. Jun 22, 2011 #9
    what i mean is how to know if when should i use the sin and cos function in computing the components of x and y :(( - sorry :(( i didnt get it :((
     
  11. Jun 22, 2011 #10
    Whenever the forces are in an arbitrary direction, you will have to use sin and cos to resolve it into two convenient directions... preferably x and y..
     
  12. Jun 22, 2011 #11
    :( yeah we will use sin and cos. but my prob. is how to determine if when to use it in my 1st post kindly take a look in x solving for the x component they use sin and cos. my prob is that . how can i determine the function to be use
     
  13. Jun 22, 2011 #12
    I had gone for lunch..

    That is simply because your prof has used a different angle from what we have discussed.

    The angle B makes is 30 with vertical. Mind it. That is 60 with horizontal..
    so it should be Fcos60 which is also same as F sin30 = F cos(90-30)

    As a thumb rule, to avoid confusion, find the angle that each force makes with the horizontal, and then use cos for x and sin for y.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Help ! can you please explain this to me ! thanks ! (it is all about vector )
  1. Can you help me here? (Replies: 6)

Loading...