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Old Jul 19th 2017, 05:16 AM   #1
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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 09:26 AM. Reason: adding details
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Old Jul 19th 2017, 06:29 AM   #2
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You need to describe the situation much better.

Neither of the two pictures you show are in equilibrium.

Did you intend this?
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Old Jul 19th 2017, 06:58 AM   #3
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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.
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Old Jul 19th 2017, 07:04 AM   #4
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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 07:06 AM. Reason: add information
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Old Jul 19th 2017, 07:09 AM   #5
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Originally Posted by osalselaka View Post
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?
There is no point asking questions about diagrams which are impossible.

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?
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Old Jul 19th 2017, 07:25 AM   #6
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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?
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Old Jul 19th 2017, 08:09 AM   #7
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Try redrawing or describing in words what you actually mean.

Your drawings do not describe what you are saying.
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Old Jul 19th 2017, 09:45 AM   #8
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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.
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Old Jul 20th 2017, 09:19 AM   #9
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Sorry, there was a mistake in my picture before and I have edited the question in above again.
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Old Jul 20th 2017, 10:23 AM   #10
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Originally Posted by osalselaka View Post
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?


Woody has partly answered this already.


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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