Calculating Member Forces of a Crane Lifting a 125kg Motor

[raymanmusic]

Homework Statement

This crane lifts a 125 kg heavey motor. What is the member forces of BF and BD?
Use 10 m/s^2 as accleration of gravity.

https://dl.dropboxusercontent.com/u/11241083/problemtruss.png

Homework Equations

F=ma
Sum Fx=0
Sum Fy=0
Sum M=0

The Attempt at a Solution

Answers:
BF force: -1939 N
BD force: -2596 N

 

[SteamKing]

Drawing a free body diagram? Writing equations of equilibrium? Bueller? Bueller?

 

[raymanmusic]

Force in G is F=125 kg * 10 m/s^2=1250 N
I found reaction force in A: 1250 N and reaction force in C: 0 N

 

[SteamKing]

Yeah, but that's not what the problem solution requires. It also ignores the weight of the lifting mechanism, which is not given, in any event. You are asked to find the forces in two members BD and BF while the motor is suspended.

Again, draw a FBD and write equations of equilibrium.

 

[raymanmusic]

At what joint do I sum moments? Do I sum x and y forces including the BD and BF forces? Can I use method of joints? I'm unable to solve this. https://dl.dropboxusercontent.com/u/11241083/New%20Document20140131230411543.pdf

 

[SteamKing]

You've got to split the hoist into several parts, and solve each individually.

For the force in member BF, start with isolating member BF and the top piece EFG. You know the load applied at point G and you know this member is supported by BF and member CDE. You can use the geometry of the hoist to supply the unknown reactions which keep EFG in equilibrium and then solve for their magnitudes.

Once you have worked the top member, take a similar approach to analyze the forces in member BD.

 

[raymanmusic]

I got the right answer for BF. How do I find BD? Do I isolate member EDC and BD and use equations of equilibrium? How do I calculate reaction force in C

 

[SteamKing]

You repeat the process used to solve member EFG. Use the reactions from that member and isolate EDC. Draw its FBD and write a new set of equilibrium equations. Remember, the joint at C is pinned, so it can't support/resist a moment.

 

[raymanmusic]

Do I solve sum of moments about joint C is equal to zero?
https://dl.dropboxusercontent.com/u/11241083/New%20Document20140201213045922.pdf

 

[SteamKing]

Yes. The joint at C is pinned and therefore the moment there must be zero.

 

[raymanmusic]

I'm unable to find the force in member BD. Can you please help me draw a FBD?

 

[nvn]

raymanmusic: I think your answers in post 1 currently look incorrect. Your reaction forces in post 3 currently look correct. Isolate member CDE, and draw all forces acting on member CDE, in a member CDE free-body diagram. Show us what your attempted member CDE free-body diagram looks like. List the value of any forces on member CDE you already know. Then, sum moments on member CDE, about point C, and show us your attempted solution for the force in member BD. Or alternately, post your answers for members BF and BD; if your answers are already correct, then you are done.

 

[raymanmusic]

My answers:
BF = -1938.87 N
BD = -2601.27 N
My attempt at solution:https://dl.dropboxusercontent.com/u/11241083/New%20Document20140202212736894.pdf

Do I only draw member CDE or do I draw EFG and CDE and then solve sum of moments about joint C equal to zero?

 

[nvn]

Post 1 says use g = 10 m/s^2.

 

[raymanmusic]

I'm sorry that is a mistake. Use g=9.81 m/s^2

 

[nvn]

raymanmusic: Your answers in post 13 are correct.

Do I only draw member CDE, or do I draw EFG and CDE, and then solve sum of moments about joint C equal to zero?

Either way is fine. Either way you prefer.