thenoname
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for example

if a sign is hanging off a 12m wooden beam, one end is P and the other end is Q.

The sign hangs 8m from P.

calculate moment of the weight of mass about P and Q

SO FOR THIS I worked it out all correctly where moment about P and Q is 313.6Nm AND 156.8Nm respectively.

what is confusing me is that; wouldn't the moments on each side be equal as the wooden beam is in equilibrium????
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Pangol
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(Original post by Riannnne)
for example

if a sign is hanging off a 12m wooden beam, one end is P and the other end is Q.

The sign hangs 8m from P.

calculate moment of the weight of mass about P and Q

SO FOR THIS I worked it out all correctly where moment about P and Q is 313.6Nm AND 156.8Nm respectively.

what is confusing me is that; wouldn't the moments on each side be equal as the wooden beam is in equilibrium????
If the beam is in equilibrium, then the sum of the clockwise moments of the forces acting on it will be equal to the sum of the anticlockwise moments of the forces acting on it, about some reference point. You are only considering the moment of one of the forces acting on the beam, and you have used two different reference points.
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mqb2766
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(Original post by Riannnne)
for example

if a sign is hanging off a 12m wooden beam, one end is P and the other end is Q.

The sign hangs 8m from P.

calculate moment of the weight of mass about P and Q

SO FOR THIS I worked it out all correctly where moment about P and Q is 313.6Nm AND 156.8Nm respectively.

what is confusing me is that; wouldn't the moments on each side be equal as the wooden beam is in equilibrium????
P & Q must be supported (?), and their reactions would be determined by / in proportion to the distance of the sign from the points.
As an extreme example, the sign could be hanging off P and the beam has zero mass. The reaction at P would take all the weight of the sign and the reaction at Q would be zero. This is exactly what the moments about P & Q would tell you.
Last edited by mqb2766; 11 months ago
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