Calculating Bending Moments for a Round Bar: E=200x103 N/mm2, D=125mm

  • Thread starter Thread starter Confusedbiomedeng
  • Start date Start date
  • Tags Tags
    Bending Moments
Click For Summary

Discussion Overview

The discussion revolves around calculating the bending moments for a round bar with a specified diameter and Young's modulus. Participants are attempting to solve a homework problem involving maximum allowable bending moment and radius of curvature, while addressing issues related to their calculations and the application of relevant equations.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant presents an attempt at solving for the maximum allowable bending moment and radius of curvature, using the equations σ/y=m/I=E/R and I=πD4/64.
  • Another participant suggests adding units to the calculations for clarity.
  • Concerns are raised about the origin of the value 17.5, with some participants guessing it represents σ, and a request for clarification on its source.
  • There is a discussion about whether the equation for the moment of inertia should be I=πD4/32 instead of I=πD4/64.
  • One participant confirms the calculation of the bending moment and the factor of 64 in the denominator, while proposing an alternative equation for the radius of curvature: R=Er/σ.
  • Another participant questions why the alternative equation for radius of curvature is preferred over the original equation M/I=E/R.
  • Responses indicate a preference for different methods of calculation without reaching a consensus on which is superior.

Areas of Agreement / Disagreement

Participants express differing views on the equations used for calculations and the interpretation of certain values. There is no consensus on the best approach or the correctness of the calculations presented.

Contextual Notes

Some participants note missing assumptions, such as the tensile strength, and the need for clarity in the calculations, particularly regarding units and the origin of specific values.

Confusedbiomedeng

Homework Statement


a round bar,125mm in diameter , is to be used as a beam. if youngs modulus for the material E=200x103 N/mm2
Calculate:
i)Maximum allowable bending moment(12marks)
ii)The radius of curvature at the point of maximum bending moment(8marks)

Homework Equations


σ/y=m/I=E/R
I=πD4/64[/B]
y=125/2

The Attempt at a Solution


i)17.5/62.5=M/11984224
0.28x11984224=3355582.72Nmm=M
ii)0.28/200x103=1.4mm=R

Can anyone tell me where I am going wrong ?
 
Physics news on Phys.org
Confusedbiomedeng said:

Homework Equations


σ/y=m/I=E/R
I=πD4/64[/B]
y=125/2

The Attempt at a Solution


i)17.5/62.5=M/11984224
0.28x11984224=3355582.72Nmm=M
ii)0.28/200x103=1.4mm=R

Can anyone tell me where I am going wrong ?
Can you put some units onto your workings out? It would be easier to follow what you are doing (for us and for you).
 
scottdave said:
Can you put some units onto your workings out? It would be easier to follow what you are doing (for us and for you).
in the final answers i have them put in ?? should i put them in throughout the workings??
 
Confusedbiomedeng said:
in the final answers i have them put in ?? should i put them in throughout the workings??
It would help me to figure out what you were doing.
 
You left out the tensile strength in the problem statement.
 
  • Like
Likes   Reactions: scottdave
Confusedbiomedeng said:
y=125/2

The Attempt at a Solution


i)17.5/62.5=M/11984224
0.28x11984224=3355582.72Nmm=M
ii)0.28/200x103=1.4mm=R

Can anyone tell me where I am going wrong ?
So where did the 17.5 come from? I am guessing it is σ. It would be nice to eliminate guessing, when figuring out where you are going wrong.
 
scottdave said:
So where did the 17.5 come from? I am guessing it is σ. It would be nice to eliminate guessing, when figuring out where you are going wrong.
Yes it is stated in the question and I forgot to put it in so 17.5n/mm2 then y is 125/2 because everything is in mm2 so I don't need to change the diameter . I ,I worked out by ∏(1254)/64 but now I'm wondering should the equation for I be ∏(D4)/32?? So sorry for any confusion
 
I confirm you calculation of the bending moment, and I confirm the factor of 64 in the denominator. But the equation for the radius of curvature should be $$R=\frac{Er}{\sigma}$$where r is the radius of the bar.
 
  • Like
Likes   Reactions: scottdave
Chestermiller said:
I confirm you calculation of the bending moment, and I confirm the factor of 64 in the denominator. But the equation for the radius of curvature should be $$R=\frac{Er}{\sigma}$$where r is the radius of the bar.
Ok could you just explain why that equation and not M/I =E/R? And so I have the right answer for the allowable bending moment
 
  • #10
Confusedbiomedeng said:
Ok could you just explain why that equation and not M/I =E/R?
It's that too. I just found it easier to calculate it the other way.
 

Similar threads

Replies
3
Views
2K
Replies
13
Views
23K
  • · Replies 15 ·
Replies
15
Views
6K
  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 8 ·
Replies
8
Views
4K
  • · Replies 3 ·
Replies
3
Views
7K
Replies
62
Views
24K
  • · Replies 2 ·
Replies
2
Views
11K
  • · Replies 1 ·
Replies
1
Views
13K
Replies
3
Views
3K