Beam calculation with 3 unknowns

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Homework Help Overview

The problem involves a 5m metal beam supported at two points, with a total weight of 30kg for the beam and an additional 65kg for a man standing at one end. The task is to determine the reaction forces at the supports and the weights added to prevent tipping, considering the acceleration due to gravity.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants discuss the equations of equilibrium, including the sum of forces and moments. There is uncertainty regarding how to handle three unknowns in the equations. Some participants suggest assuming the added weight at B is the minimum necessary to prevent tipping, which may simplify the calculations.

Discussion Status

Participants are exploring different interpretations of the problem setup, including the positioning of the man and the beam's length. Some have provided insights into the torque calculations, while others are questioning the accuracy of the figures presented. There is no explicit consensus on the approach yet, but hints towards a potential direction have been offered.

Contextual Notes

There is a noted discrepancy regarding the beam's length and the segments shown in the problem, leading to questions about the accuracy of the measurements provided. Participants are also considering the implications of the man's position relative to the supports.

willowdanny
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Homework Statement


A 5m metal beam is supported on 2 supports. The beam is 30kg and the man is 65kg. The man stands at A, to prevent the beam tipping extra weights have been placed at B. Find reaction forces and the weights that have been placed at B. Take acceleration due to gravity to be 9.8ms
moment.png

Homework Equations


Force * Distance
Sum of Clockwise = Sum of Anti-Clockwise
Sum of Downward Forces = Sum of Upward Forces

The Attempt at a Solution


Decorator - 637N
Beam - 294N
F1 being the first support reaction force
F2 being the second reaction force
W being the extra weights added

F1 + F2 = 931 + W (Sum of up and down)

3F1 + F2 = 3601.5 (sum of clockwise/anticlockwise)

I have attempted so many different ways to figure this out, but I have no idea how to get around having 3 unknowns at a time. When I take a moment around one of the supports, I have the reaction force the other plus the weight being placed at B. I'm not sure if I'm missing something obvious or not.
Thanks in advance for literally any help
 

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Welcome to PF!

Maybe you are supposed to assume that the weight at B is the minimum weight that will prevent the beam from tipping.

I don't quite agree with your number 3601.5 in the torque equation. EDIT: Nevermind, I see that A is not at the end of the beam. The number 3601.5 looks good.
 
Last edited:
willowdanny said:

Homework Statement


A 5m metal beam is supported on 2 supports. The beam is 30kg and the man is 65kg. The man stands at A, to prevent the beam tipping extra weights have been placed at B. Find reaction forces and the weights that have been placed at B. Take acceleration due to gravity to be 9.8ms
View attachment 226011

Homework Equations


Force * Distance
Sum of Clockwise = Sum of Anti-Clockwise
Sum of Downward Forces = Sum of Upward Forces

The Attempt at a Solution


Decorator - 637N
Beam - 294N
F1 being the first support reaction force
F2 being the second reaction force
W being the extra weights added

F1 + F2 = 931 + W (Sum of up and down)

3F1 + F2 = 3601.5 (sum of clockwise/anticlockwise)

I have attempted so many different ways to figure this out, but I have no idea how to get around having 3 unknowns at a time. When I take a moment around one of the supports, I have the reaction force the other plus the weight being placed at B. I'm not sure if I'm missing something obvious or not.
Thanks in advance for literally any help

You say the beam is 5m in length, but the segments shown add up to only 4.5m. Which figure is erroneous?
 
Ray Vickson said:
You say the beam is 5m in length, but the segments shown add up to only 4.5m. Which figure is erroneous?
I overlooked that A is not at the end of the beam. So, the beam can be 5 m long.
 
Ray Vickson said:
You say the beam is 5m in length, but the segments shown add up to only 4.5m. Which figure is erroneous?
The beam is 5m in length, the man is standing 0.5m away from the end of the beam.
 
willowdanny said:
The beam is 5m in length, the man is standing 0.5m away from the end of the beam.
Have you tried assuming the added weight is the minimum, as TSny suggests? That is also how I read the question. Hint: it allows you to write down one reaction force immediately.
 

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