[craglyboy]

Uploaded with ImageShack.us

Can anyone help me with these moments I take about A and get different results to taking them about B

[bcrazy]

Are you considering the reaction forces at A and B? If so, can you show what working you have done?

[craglyboy]

(Original post by bcrazy)
Are you considering the reaction forces at A and B? If so, can you show what working you have done?

Yeah trying to find RA and RB I call em Ay and By
sum of moments about B=0

(-3x1)+(5x2)-(Ayx3)+(4x4)

Ay=7.6KN

but taking moments about A I get By=1KN and then Ay=5KN

[bcrazy]

(Original post by craglyboy)
Yeah trying to find RA and RB I call em Ay and By
sum of moments about B=0

(-3x1)+(5x2)-(Ayx3)+(4x4)

Ay=7.6KN

but taking moments about A I get By=1KN and then Ay=5KN

You need to check your working when taking moments about A...show it if you can't find your mistake. You should find that the force at B is actually acting downwards.

[craglyboy]

(Original post by bcrazy)
You need to check your working when taking moments about A...show it if you can't find your mistake. You should find that the force at B is actually acting downwards.

So my working for taking moments about B is correct?

For moments about A I got:

(4x1)+(5x1)-(3x2)-(3xBy)=0

to get By=1KN

[bcrazy]

(Original post by craglyboy)
So my working for taking moments about B is correct?

Yes

[craglyboy]

(Original post by bcrazy)
Yes

For moments about A I got:

(4x1)+(5x1)-(3x2)-(3xBy)=0

to get By=1KN

but you say this is wrong so is By from equating the forces using Ay=7.6Kn

Sum of Fx=4-7.6+5-3-By=0

By=-1.6Kn

[bcrazy]

(Original post by craglyboy)
For moments about A I got:

(4x1)+(5x1)-(3x2)-(3xBy)=0

to get By=1KN

If you are taking clockwise as positive then then 4kN force should be negative (make sure you always think carefully in terms of clockwise and anticlockwise rather than up and down)

[craglyboy]

(Original post by bcrazy)
If you are taking clockwise as positive then then 4kN force should be negative (make sure you always think carefully in terms of clockwise and anticlockwise rather than up and down)

Is the bottom part of my last post wrong then as well

For moments about A I got:

(4x1)+(5x1)-(3x2)-(3xBy)=0

to get By=1KN

but you say this is wrong so is By from equating the forces using Ay=7.6Kn

Sum of Fx=4-7.6+5-3-By=0

By=-1.6Kn

[bcrazy]

(Original post by craglyboy)
Is the bottom part of my last post wrong then as well

For moments about A I got:

(4x1)+(5x1)-(3x2)-(3xBy)=0

to get By=1KN

but you say this is wrong so is By from equating the forces using Ay=7.6Kn

Sum of Fx=4-7.6+5-3-By=0

By=-1.6Kn

Sorry, have got confused with your edits which bit you are asking about at the start but your working giving By=-1.6Kn is correct (ignoring the slightly dodgy rounding you did earlier on...should be Ay=23/3 and By=-5/3)

[craglyboy]

Taking anti-clockwise as positive in all this I hope

[craglyboy]

(Original post by bcrazy)
Sorry, have got confused with your edits which bit you are asking about at the start but your working giving By=-1.6Kn is correct (ignoring the slightly dodgy rounding you did earlier on...should be Ay=23/3 and By=-5/3)

I was on about resolving the Fx, thanks for the help I think my working is all alright now apart from shoddy rounding as you say.

Can you show me how you would take moments about A?

[bcrazy]

(Original post by craglyboy)
Taking anti-clockwise as positive in all this I hope

Sorry, yes! I would have taken moments about A the same as you did when you corrected it...but actually I would have only looked to take moments about B and then resolved vertically- the only reason I would be tempted to take moments about A would be to double check.

[craglyboy]

(Original post by bcrazy)


Sorry, yes! I would have taken moments about A the same as you did when you corrected it...but actually I would have only looked to take moments about B and then resolved vertically- the only reason I would be tempted to take moments about A would be to double check.

Yeah that was the only reason I did to wasn't very confident and seemingly rightly so thanks anyways now I can start to answer the question, got to find shear force and bending moment now haha thanks man!