Solution to Moments Question: Find 11 N Force

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In summary: This is because the question asks for the moment about the pivot, and the center of mass is over the pivot in the first diagram.In summary, the homework equation states that the center of mass of the retort stand is over the pivot point.
  • #1
hasnain721
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Homework Statement



http://img510.imageshack.us/img510/7663/26160035im5.jpg

Homework Equations



principle of moments

The Attempt at a Solution



I took moments about the base of teh retort stand :
(220 X x) = 228 X 9

x = 9.33 which is wrong as the question says show that it is 11 Newtons.
 
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  • #2
Where's the center of mass of the stand?
 
  • #3
Doc Al said:
Where's the center of mass of the stand?


in the middle i believe where the pivot it
 
  • #4
I mean the center of mass of the retort stand alone. (Its center of mass cannot be over the pivot, otherwise it would tip over when the weight is added.)
 
  • #5
Doc Al said:
I mean the center of mass of the retort stand alone. (Its center of mass cannot be over the pivot, otherwise it would tip over when the weight is added.)

i thougt that the centre of mass is where the retort stand balances ..this is the full question :


http://img218.imageshack.us/img218/8936/15977211jl9.jpg
 
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  • #6
Yes, in this diagram--given in part (b)--the center of mass is over the pivot point. :wink:

Now use that information when you find the moments in part (i).
 
  • #7
Doc Al said:
Yes, in this diagram--given in part (b)--the center of mass is over the pivot point. :wink:

Now use that information when you find the moments in part (i).

rite...moments is when u multiply the force by the perpendicular distance:

moment about the pivot :


(220 X x) = (448-220) X 9



this does not work!
 
  • #8
There are two forces that create torques on this thing. One is its weight. Where does that act? (Hint: Not at 220 mm from the pivot! Hint 2: Where's the center of mass of the retort stand?)
 
  • #9
Doc Al said:
There are two forces that create torques on this thing. One is its weight. Where does that act? (Hint: Not at 220 mm from the pivot! Hint 2: Where's the center of mass of the retort stand?)

the weight acts through the pivot and if we r taking moment about the pivot, i believe that it does not have a moment rite?
 
  • #10
The weight of any object acts through its center of mass. Use the first diagram to find the center of mass of this object. (How far is the center of mass from the left end?)
 
  • #11
Doc Al said:
The weight of any object acts through its center of mass. Use the first diagram to find the center of mass of this object. (How far is the center of mass from the left end?)

220 mm ?
 
  • #12
hasnain721 said:
220 mm ?
I'm talking about the first diagram in your latest scan--the one marked part (b). 220 mm doesn't appear anywhere on that diagram.
 
  • #13
Doc Al said:
I'm talking about the first diagram in your latest scan--the one marked part (b). 220 mm doesn't appear anywhere on that diagram.

ah...40mm
 
  • #14
hasnain721 said:
ah...40mm
Right. That diagram tells you where the center of mass of the object is. Now use that fact when you analyze the forces in part (i). Hint: How far is the center of mass from the pivot in that second diagram?
 
  • #15
Doc Al said:
Right. That diagram tells you where the center of mass of the object is. Now use that fact when you analyze the forces in part (i). Hint: How far is the center of mass from the pivot in that second diagram?

srry I'm tryin my best...i'm rubbish at moments any ways :


if we take moments about the centre of mass : (40 X x) + ((448 - 40) X 9 ) = 0

stil doesn't work.
 
  • #16
Take moments about the pivot point, not the center of mass. (Since the weight acts at the center of mass, it cannot exert a moment about the center of mass!)
 
  • #17
Doc Al said:
Take moments about the pivot point, not the center of mass. (Since the weight acts at the center of mass, it cannot exert a moment about the center of mass!)

ok...if we take moments about the pivot then :

(40 X x) + (228 X 9 ) = 0 ?
 
  • #18
hasnain721 said:
ok...if we take moments about the pivot then :

(40 X x) + (228 X 9 ) = 0 ?
Two problems:
(1) The distance between center of mass and pivot point is not 40. (That's the distance between center of mass and the left end.)
(2) Since one moment is clockwise and the other counterclockwise, they must have different signs.
 
  • #19
Doc Al said:
Two problems:
(1) The distance between center of mass and pivot point is not 40. (That's the distance between center of mass and the left end.)

the centre of mass is at 40 mm and the pivot is 220 mm. Therefore the distance between them should be 220 - 40 = 180 mmm

Doc Al said:
(2) Since one moment is clockwise and the other counterclockwise, they must have different signs.
yea clockwise moments = anticlock wise moments

(180 X x) = ((448 - 220) X 9 )

180 x = 228 X 9

180 x = 2052
x = 11.4 !IT WORKS!THANK U SOOO MUCH!
 
  • #20
man u explain every thing so well... ifu really dun mind can u explain the next one please :
http://img528.imageshack.us/img528/7795/43744261tf4.jpg i don't see how force and weight are related?
 
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  • #21
The object is now balanced on its left end, so take moments about that point. Where does its weight act? (Same place as always: Its center of mass.) The "Newtonmeter" just provides an upward force to balance things. Set up your moment equation and you can solve for that force. (Since you know the weight of the object now.)
 
  • #22
Doc Al said:
The object is now balanced on its left end, so take moments about that point. Where does its weight act? (Same place as always: Its center of mass.) The "Newtonmeter" just provides an upward force to balance things. Set up your moment equation and you can solve for that force. (Since you know the weight of the object now.)

so if we take moments about the left end then :


(40 X 11.4) = ( 448 X x)

x = 0.991

x = 1 N is that rite?
 
  • #23
Looks good to me. (My calculator gives an answer slightly different from 0.991--but close enough!)
 
  • #24
Doc Al said:
Looks good to me. (My calculator gives an answer slightly different from 0.991--but close enough!)

thank u veeeeeeeeery much...would u recommend any websites which have questions on moments as i am fairly weak in that area. I really appreciate your efforts in volunteering to help dumb people like me.
 
  • #26
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1. How do I find the solution for the moment question involving an 11 N force?

To find the solution for this moment question, you will need to use the formula for moment, which is force multiplied by the perpendicular distance from the point of rotation. In this case, you will need to multiply 11 N by the perpendicular distance from the point of rotation to the line of action of the force.

2. What is the unit of measurement for the solution to this moment question?

The unit of measurement for the solution to this moment question will be Newton-meters (Nm). This is a unit of torque, which is the measure of a force's ability to rotate an object.

3. Can you provide an example of how to calculate the solution for this moment question?

Sure, let's say the 11 N force is acting at a distance of 2 meters from the point of rotation. To find the moment, we would multiply 11 N by 2 meters, giving us a moment of 22 Nm.

4. How does the direction of the force affect the solution to this moment question?

The direction of the force does not affect the solution to this moment question. The moment is only dependent on the magnitude of the force and the perpendicular distance from the point of rotation to the line of action of the force.

5. What is the importance of finding the solution to this moment question?

The solution to this moment question is important because it allows us to determine the rotational effect of a force on an object. This knowledge is essential in many areas of science and engineering, such as designing structures and machines.

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