Calculating Member Forces of a Crane Lifting a 125kg Motor

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Discussion Overview

The discussion revolves around calculating the member forces of a crane lifting a 125 kg motor. Participants explore the application of static equilibrium equations and free body diagrams to determine the forces in specific members of the crane structure.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant states the forces in members BF and BD as -1939 N and -2596 N, respectively, based on their calculations.
  • Another participant suggests drawing a free body diagram and writing equations of equilibrium as essential steps in solving the problem.
  • A participant calculates the force in member G as 1250 N and identifies reaction forces at points A and C.
  • Concerns are raised about the initial calculations, noting that they do not account for the weight of the lifting mechanism, which is unspecified.
  • Questions arise about the appropriate joints for summing moments and whether to include forces from members BD and BF in the equilibrium equations.
  • One participant recommends isolating member BF and using the geometry of the hoist to find unknown reactions for equilibrium analysis.
  • Another participant confirms the need to isolate member EDC and suggests using equilibrium equations to find the force in member BD.
  • Clarifications are made regarding the pinned joint at C, emphasizing that it cannot resist a moment.
  • Participants discuss the necessity of drawing free body diagrams for different members and summing moments to solve for forces.
  • One participant expresses uncertainty about their calculations and seeks assistance in drawing a free body diagram.
  • Another participant questions the correctness of earlier answers and suggests isolating member CDE to analyze forces further.
  • Later contributions indicate a correction in the gravitational constant used, changing from 10 m/s² to 9.81 m/s².
  • Finally, a participant confirms the correctness of their answers while seeking clarification on the approach to drawing diagrams and summing moments.

Areas of Agreement / Disagreement

Participants express differing views on the calculations and methods to approach the problem. There is no consensus on the correctness of the initial answers, and multiple approaches to solving the problem are discussed.

Contextual Notes

Some participants note the importance of considering the weight of the lifting mechanism, which is not provided. There are also unresolved questions regarding the correct application of equilibrium equations and the specific forces acting on various members.

raymanmusic
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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 acceleration 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
 
Last edited by a moderator:
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Drawing a free body diagram? Writing equations of equilibrium? Bueller? Bueller?
 
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
 
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.
 
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
 
Last edited by a moderator:
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.
 
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
 
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.
 
Do I solve sum of moments about joint C is equal to zero?
https://dl.dropboxusercontent.com/u/11241083/New%20Document20140201213045922.pdf
 
Last edited by a moderator:
  • #10
Yes. The joint at C is pinned and therefore the moment there must be zero.
 
  • #11
I'm unable to find the force in member BD. Can you please help me draw a FBD?
 
  • #12
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.
 
Last edited:
  • #13
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?
 
Last edited by a moderator:
  • #14
Post 1 says use g = 10 m/s^2.
 
Last edited:
  • #15
I'm sorry that is a mistake. Use g=9.81 m/s^2
 
  • #16
raymanmusic: Your answers in post 13 are correct.

raymanmusic said:
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.
 
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