[static equilibrium] pin and link support type

  1. Hello every one

    I am confused between two support type:
    the pin, and the link

    On the textbook, it says, the pin prevents the object from translating vertically and horzontally, therefore, it provides fx and fy reaction forces.

    but another type of support is much identical to the pin, which is the link support, the photo is here: http://imageshack.us/f/105/pinconnectiontw4.jpg/

    at point A, the beam AB is subjected to the pin support, so it has two components of reaction forces, namely fx and fy

    but what about at point G, how come it only provides one force components, namely fx, but not fy as well? is the point G also classified as pin support?
    i am very confused; instructor said that whenever u see a pin support, there's two force components; how come this is not a case here?
  2. jcsd
  3. Re: [engineer need help] to understand support type of static equlibrium

    is the link support the beam which connects from GB, and pin support the node at G and A?
  4. hotvette

    hotvette 931
    Homework Helper

    Re: [engineer need help] to understand support type of static equlibrium

    G is a pinned support as well as A. The reason there is reaction in x only at G is because member GB can support axial loads only.
  5. PhanthomJay

    PhanthomJay 6,334
    Science Advisor
    Homework Helper
    Gold Member

    Yes, point G is a pin support. In general, there are 2 force components at a pin, in the x and y direction, but in some cases, such as joint G, there is only one. This is due to equilibrium considerations for so called '2-force' members. A 2-force member is a member subjected to a force (applied or a reaction force) at each end with no forces applied in between. All members of a truss are 2 force members when loads are applied at the joints only. Member BG is a 2-force member. As such, it can only take axial loading in tension or compression along its longitudinal axis. Thus , the reaction at G must be entirely horizontal; otherwise, the resultant reaction force could not be directed along the BG axis.

    EDIT: I notice you are posting the same question in the ME sub-forum. You should stick to that forum and communicate further with the responder if you have additional questions.
    Last edited: Oct 10, 2011
  6. apologize, this is my first time to use this forum... . I will never do that again.

    and what about this http://www.ecourses.ou.edu/cgi-bin/view_anime.cgi?file=512LIN.swf&course=st&chap_sec=05.1, the link support, which is very similar to the pin support? Same reasoning as well?
  7. PhanthomJay

    PhanthomJay 6,334
    Science Advisor
    Homework Helper
    Gold Member

    Welcome to Physics forums, kougou! Yes, same reasoning for a link support; the support reactions are always directed along the longitudinal axis of the member, creating only tension or compression axial forces in the member, regardless of the angle of the link, provided that there are no forces applied in between the 2 ends. NOTE that when the link is at an angle, there are still x and y components of the link reaction forces, but their resultant sum is a single force directed axially along the link, no bending or shear stresses.
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