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Homework Help: A couple of statics questions

  1. Jul 2, 2004 #1
    Hi, I have a couple of questions from a statics assignment that I'm in need of help on. Here's what I have so far:

    Two rods are connected by a slider block as shown. Neglecting the effect of friction, determine the couple Ma required to hold the system in equilibrium.

    And this is what I believe the forces are:

    It would seem to me that the external forces are irrelevant to this problem, but I'm guessing not. What I came up with for this problem is:

    Sum of moments about B=0 => -25+Nd*271.89=0 => Nd=0.0919
    Sum of moments about A=0 => -.0919*150+Ma=0 => Ma=13.79 N*m

    I know the answer is Ma=15.22 N*m. I'm guessing that I also need to consider external forces or something.

    The next problem is

    The double toggle mechanism shown is used in a punching machine. Knowing that the links AB and BC are each of length 150 mm, determine the couple M required to hold the system in equilibrium when Phi=20 (degrees).

    Once again, my analysis of the problem is:

    Obvious things are:

    sum of moments about B of AB=0 => M+By*150*cos(30)+Bx*150*sin(30)=0

    Since AB is a two-force body, Bx=B*cos(30), By=B*sin(30) and A=-B.

    sum of moments about C of BC=0 => -B'x*150*sin(30)+By*150*cos(30)=0
    sum of moments about B of BC=0 => -Cy*150*cos(30)+Cx*150*sin(30)=0

    And so on and so forth. If I continue in this manner I figure I'll end up with something around 24 equations, there has got to be a simplier method. Any idea what that might be?

    Any hints or help would greatly be appreciated.

    Last edited by a moderator: Apr 21, 2017
  2. jcsd
  3. Jul 5, 2004 #2
    Ok, the first problem is easy, I just forgot to get a perpendicular distance to the force, so it works fine now.

    As for the second problem, I was thinking about just balancing the Fx, Fy, Ma, Md, and Me, but that doesn't seem to work either. Any suggestions would be appreciated.
  4. Jul 5, 2004 #3


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    Is [tex]N_D[/tex] supposed to be an applied force in that drawing?

    Can you list the terms you have for the net forces, and net torques, and which point they are about?
  5. Jul 7, 2004 #4
    Sorry, [tex]N_D[/tex] was an internal force from the collar. Thanks anyway, but I managed to figure this one out.
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