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Registered:
October 2004
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: HELP! Question on forces (Mechanical eng. problem!)
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Tue, 16 August 2005 15:11
 
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fakr it's been a while..
iirc, you look at each joint individually starting from the easiest 
hmmm ok.. i kinda think like it's a big floppy system that you have to make solid by putting forces in...
talking vertical forces first..
BC has to provide the "up" force to hold up the load at C...
at A, the vertical load points down to counter the force at C and load.. (as tho it's pivoting around C)
now.. at B, you have to provide the vertical force for point C, but it is at an angle, so the member is pushing into the wall, but at C, BC is trying to push the top member away from the wall..
and so A also has a horizontal component pulling ALoad back toward the wall..
is this making any sense?
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Registered:
October 2004
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Re: HELP! Question on forces (Mechanical eng. problem!)
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Tue, 16 August 2005 16:03
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so say --> is the positive direction,
you're saying the reaction force on member AB, at A, A_x is going this way <--- (negative)?
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Location:
Adelaide
Registered:
May 2002
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Re: HELP! Question on forces (Mechanical eng. problem!)
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Tue, 16 August 2005 23:12
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Hi there,
I cant see the pic unfortunately as my work server blocks almost everything...
Static problems are really simple when you just set a direction to be positive (eg up for y and right for x) and then just assign all forces a +/- symbol depending on which direction they are going, leave any unknown forces directionless. Do this for each member (ie make a FBD for each rigid part )
Then you have 3 equations, 1. sum in x direction = 0
2. sum in y direction = 0
3. sum of moments = 0
you will then know what direction your unknowns are going by solving these and looking at their sign.
This is generic but will work everytime
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Location:
Perth
Registered:
October 2003
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Re: HELP! Question on forces (Mechanical eng. problem!)
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Tue, 16 August 2005 23:42
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If you release A and make C a pin, A will obvioulsy rotate clockwise. So just to iterate oldrollas comments the reactions at A will be down and to the left.
Don't confuse yourself with whether or not the arrows are pointing at the pin cause you could place a force arrow upwards on either side of the pin, hence 1 would be pointing at the pin and the other wouldn't but the force is still in the same direction.
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Location:
Melbourne
Registered:
March 2004
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Re: HELP! Question on forces (Mechanical eng. problem!)
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Thu, 18 August 2005 06:26
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as my civil lecturer used to say:
"you must cokulate the forces numbah of johns, or you will go to jail and things like that." hahah, oh billy wong...
soz, doesn't really help you!
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Location:
Canberra
Registered:
August 2003
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Re: HELP! Question on forces (Mechanical eng. problem!)
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Thu, 18 August 2005 06:31
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Reminds me why I choose Computer Engineering 1 + 1 = 10 seems simple in comparison!!!
It also means that an 'oops' almost never results in anyone dying. 
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