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 Jul 19th 2017, 04:16 AM #1 Junior Member   Join Date: May 2017 Location: Sri Lanka Posts: 17 Moment Questions This question is according to the moment. A. Think about the door. When I add a force of 10N which has perpendicular distance of 5m from the hinge, it has 10*5=50Nm moment. The force is acting by the place where I added 10N. B. When I add a force of 10N which has perpendicular distance of 5m from the hinge, and another force of 15N is adding with no(0) perpendicular distance .Then the moment is (10*5)+(15*0). In the A and B situations the moment is equal. But I have the question that is, the place where the total force is forming is change in A and B situations? (Practically when I do this experiment I felt same the two situations and therefore why the place of total force is forming is change in the two situations?) Last edited by osalselaka; Jul 20th 2017 at 08:26 AM. Reason: adding details
 Jul 19th 2017, 05:29 AM #2 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 613 You need to describe the situation much better. Neither of the two pictures you show are in equilibrium. Did you intend this?
 Jul 19th 2017, 05:58 AM #3 Senior Member     Join Date: Jun 2016 Location: England Posts: 264 it looks as though the feature the problem is trying to illuminate is that the total forces are different for A and B, but the moment about the pivot is the same. __________________ ~\o/~
 Jul 19th 2017, 06:04 AM #4 Junior Member   Join Date: May 2017 Location: Sri Lanka Posts: 17 Reply to Studiot-These two pictures are not equilibrium. There is a resultant moment in both. But my problem is the 5N force is not affect to the moment. But according to my teacher when the 5N added then the resultant force forming place is changing. These 2 pictures momentum is equal. Therefore how the resultant forces forming place is changing? Last edited by osalselaka; Jul 19th 2017 at 06:06 AM. Reason: add information
Jul 19th 2017, 06:09 AM   #5
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 Originally Posted by osalselaka Reply to Studiot-These two pictures are not equilibrium. There is a resultant moment in both. But my problem is the 5N force is not affect to the moment. But according to my teacher when the 5N added then the resultant force forming place is changing. These 2 pictures momentum is equal. Therefore how the resultant forces forming place is changing?

If the bars are not in equilibrium, they are moving.

This movement could be a translation unless you have omitted some forces.

It could also be a rotation.

Either way how are the shown forces applied to the moving bars?

 Jul 19th 2017, 06:25 AM #6 Junior Member   Join Date: May 2017 Location: Sri Lanka Posts: 17 I drew this diagram to have a rotaion. The rotation is forming because of there is a resultant force. These two rotations should be equal because of the moment is not change. This rotating is form because of there is a resultant force. But how the resultant of parallel forces , the line of action is changing?
 Jul 19th 2017, 07:09 AM #7 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 613 Try redrawing or describing in words what you actually mean. Your drawings do not describe what you are saying.
 Jul 19th 2017, 08:45 AM #8 Senior Member     Join Date: Jun 2016 Location: England Posts: 264 Remember that if the pivot point is being held stationary, then the pivot support will be contributing a force to the system. This pivot support force is not explicitly shown in the diagram. __________________ ~\o/~
 Jul 20th 2017, 08:19 AM #9 Junior Member   Join Date: May 2017 Location: Sri Lanka Posts: 17 Sorry, there was a mistake in my picture before and I have edited the question in above again.
Jul 20th 2017, 09:23 AM   #10
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Join Date: Apr 2015
Location: Somerset, England
Posts: 613
 Originally Posted by osalselaka This question is according to the moment. A. Think about the door. When I add a force of 10N which has perpendicular distance of 5m from the hinge, it has 10*5=50Nm moment. The force is acting by the place where I added 10N. B. When I add a force of 10N which has perpendicular distance of 5m from the hinge, and another force of 15N is adding with no(0) perpendicular distance .Then the moment is (10*5)+(15*0). In the A and B situations the moment is equal. But I have the question that is, the place where the total force is forming is change in A and B situations? (Practically when I do this experiment I felt same the two situations and therefore why the place of total force is forming is change in the two situations?)
Do I understand your question to be:-

Configuration 'A' 10N acting on the door

Configuration 'B' (10N + 15 N) = 25N acting on the door

But

Why is the moment is the same in both configurations A and B?

We consider Force sums and Moment sums separately, they are independent.

In both A and B you have omitted another force, the hinge reaction.

This exactly equals the force sum (10N in A and 25N in B)
but
since it passes through the hinge it has zero moment about the hinge.

Does this help?

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