Understanding Moments: A Method for Solving Question 2.b.i.

[Magic Mushroom]

Hi, I'm having a little trouble with this simple question on moments and I'm wondering if anyone could help me out by giving me a suitable method.
http://www.aqa.org.uk/qual/gceasa/qp-ms/AQA-PHB1-W-QP-JUN05.PDF that is the link and the question is 2.b.i. Please don't give me the answer just the method! :)
Thanks in advance.

 

[radou]

Hi, I'm having a little trouble with this simple question on moments and I'm wondering if anyone could help me out by giving me a suitable method.
http://www.aqa.org.uk/qual/gceasa/qp-ms/AQA-PHB1-W-QP-JUN05.PDF that is the link and the question is 2.b.i. Please don't give me the answer just the method! :)
Thanks in advance.

The sum of the moments of all forces acting on the bar must vanish (including the weight of the bar). Use that fact to obtain the reaction in the pivot.

 

[OlderDan]

The sum of the moments of all forces acting on the bar must vanish (including the weight of the bar). Use that fact to obtain the reaction in the pivot.

This will work for 2.b.i if the moments are calculated about some point other than the pivot. Perhaps the OP is having difficulty because questions 2.a has them thinking only of moments about the pivot.

Alternatively, once the weight of the beam is known from 2.a.iii the reaction in the pivot can be found by summing all the forces to get zero.

 

[radou]

This will work for 2.b.i if the moments are calculated about some point other than the pivot. Perhaps the OP is having difficulty because questions 2.a has them thinking only of moments about the pivot.

Alternatively, once the weight of the beam is known from 2.a.iii the reaction in the pivot can be found by summing all the forces to get zero.

Of course, I forgot to mention that the point of reduction must not be taken at the pivot. Take the point at the end of the bar and set the sum of all moments equal to zero. Further on, after getting the reaction force of the pivot, you may check on your calculation by setting the sum of all forces (not moments) equal to zero.