# Fiding resultant moment about a given point?

1. Oct 23, 2005

### Ancient

Hello!
I have started with AS level physics this school year and our teacher has set us some prep over half term and there is one question on there that I do not understand.
I have attached the question to this post.
The problem with this question is that it is the first question he ever set us where forces are not acting up or down.
I kow from GCSE that moment = Force * perpendicular distance
Now the problem for me with the question is the force that acts on the disk horizontaly since the perpendicular distance is not given. I have considered using trigonometry to get the perpendicular distance but for some reason i get really weird answers.
I hope some one here can help me with this question.

Edit:

I forgot to show the direction of the vertical force on my drawing it is upwards

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Last edited: Oct 23, 2005
2. Oct 23, 2005

### Staff: Mentor

To find the perpendicular distance try extending the line of the horizontal force to see how close it gets to the center. (The answer is simpler than you think.)

3. Oct 23, 2005

### Ancient

Hi and thx for helping Doc Al!

I i extended the line but unfortunatly that does not help me.

unless the answer is that it is 2x 0.24 (0.48) ?

This question comes from a past paper and is worth 3 marks so i dont tihink that the answer is that easy.

could you try explaiing your point to me a bit clearer ?

thanks Ancient.

4. Oct 23, 2005

### Staff: Mentor

I assume the problem is that there are two forces acting on the edge of the disk: one horizontal, the other vertical. I further assume that you know how to find the moment generated by the vertical force. So the only problem is the horizontal force. Correct?

Did you draw the line of the horizontal force? How close does it come to the center? The perpendicular distance is the distance between the line of the horizontal force and a point (the center).

5. Oct 23, 2005

### Ancient

Ok I think i have got it now:

Resultant moment about O = Sqrt(F1^2 + F2^2).
F1 = 5.0N * 0.24m = 1.2Nm , F2 = 5.0N * 0.24m = 1.2Nm

is that correct ?

I am sorry if my questions are dumb but we just started momets 2 lessons ago and we have got half term no so I can ot ask my teacher.

Thanks again for your patcience and help.

Ancient.

Last edited: Oct 23, 2005
6. Oct 23, 2005

### Staff: Mentor

No. Did you draw the line of the horizontal force to see its distance from the center?

Here's a hint: Imagine the disk is a steering wheel. If you apply the vertical force, will the wheel tend to turn? What if you applied the horizontal force?

7. Oct 23, 2005

### Ancient

Ok so can i simply ignore the horizontal force for the purpose of determining the moment ?
And what do you mean by drawing line of horizontal force ?
Is the distance not the 0.24m ?

8. Oct 23, 2005

### Staff: Mentor

Its moment is zero!
To calculate the moment of a force about an axis, you need the perpendicular distance, not just the distance. To find the perpendicular distance, extend the line of force and find the line from the center that is perpendicular to the line of force. If you draw the line of the horizontal force, it passes right through the center. The perpendicular distance is zero!

You might find this helpful: http://hyperphysics.phy-astr.gsu.edu/hbase/torq.html

9. Oct 24, 2011

### umass

Hi guys, I am trying to find a resultant moment on the circle relatively a centerline of the circle. Centerline is perpendicular to the surface of the circle. I have two forces perpendicular to the circle. The point of the forces locations are 0 degree and 90 degrees (0.25 circumference of a circle). Any thought how to find resultant moment or resultant force or location of the resultant force? Thanks