Solving Simple Truss Problem: Find Reaction Force at A

[NoobeAtPhysics]

Homework Statement

The truss is suspended by two pin joints at A and B. Each of the four segments is 3 m wide and 4 m high. Find the reaction force at A

Homework Equations

d

The Attempt at a Solution

sum of forces y = -ay + by -f=0

Can someone tell me what the moment at B is?

Mb = -4*w*F + 0*Ay - Ax*h = 0

i don't know if that is right. Ay and ax confuse me

 

[NoobeAtPhysics]

Hmm I got that Ax = -3F but I can't figure out what Ay is

 

[NoobeAtPhysics]

Any thoughts?

 

[NoobeAtPhysics]

Hmm, a friend told me Ay = F but.. I don't understand why?

 

[voko]

Once you know Ax, and, then, Bx, consider the moments of the forces about the point where F is applied. That will give you an equation linking Ay, Bx, and By.

 

[NoobeAtPhysics]

Thank you

but I don't know what Ay and By are, so that wouldn't help me would it?

 

[voko]

You have an equation that relates them.

You have posted another equation with them in #1.

Two equations, two unknowns, problem solved.

 

[PhanthomJay]

Noobee,
Are you sure that both joints are pinned hinges and that there is not a roller support at B that would make it incapable of supporting vertical load? If the support at B was a roller support , then Ay = F. But if both A and B supports can support vertical loads, then the problem is statically indeterminate because you have 4 unknowns Ax Bx Ay and By, and only 3 equilibrium equations. It only affects the forces in AB, in either case. You may assume Ay = By = F/2, if both supports are full hinges capable of taking loads in the x and y directions.

 

[voko]

But if both A and B supports can support vertical loads, then the problem is statically indeterminate because you have 4 unknowns Ax Bx Ay and By, and only 3 equilibrium equations.

Yes indeed. Using my "method", I end up having two identical equations, as was to be expected if I had thought straight. Not sure what I was thinking.

 

[NoobeAtPhysics]

"Ay = By = F/2"

but Ay is -3F and By = 3F (which is the correct answer)

Also there are no rollers in this problem only pins.

 

[PhanthomJay]

"Ay = By = F/2"

but Ay is -3F and By = 3F (which is the correct answer)

Also there are no rollers in this problem only pins.

That is not the correct answer for Ay and By. That is the correct answer for Ax and Bx (you correctly solved for Ax in your 2nd post). Note: It is assumed that x is the horizontal axis and y is the vertical axis.

Did you state the problem exactly as written?