Moment of force and the meter rule

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Discussion Overview

The discussion revolves around a physics problem involving a metre rule balanced on a knife-edge, focusing on calculating moments and the mass of the rule based on its center of gravity. Participants explore the application of the moment of force concept in a practical scenario.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant states the formula for turning moment as Force x Distance from the pivot and calculates the moment based on a weight of 0.45 N and a distance of 25 cm.
  • Another participant confirms the calculation but questions the units used, suggesting the need for standard units.
  • A participant suggests converting the distance to meters for SI unit consistency, resulting in a moment of 0.1125 Nm.
  • Further, a participant proposes using clockwise and anticlockwise moments to solve for the mass of the rule, indicating the need for an equation with one unknown.
  • Another participant calculates the mass using the distances from the pivot and the weight, arriving at a mass of 0.023 kg, but questions the correctness of their approach.
  • There is a discussion about the distances used in calculations, with one participant asking for clarification on the origin of a specific distance value.

Areas of Agreement / Disagreement

Participants express differing views on the calculations and the correct application of the moment of force concept. There is no consensus on the final mass calculation or the distances used in the equations.

Contextual Notes

Some participants note the importance of using consistent units and the relevance of distances from the pivot point in moment calculations. There are unresolved aspects regarding the application of the center of gravity in the context of the problem.

Googl
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Hi,

I have a revision question, it's actually quite popular on the Internet but I would just like some help with it.

A metre rule is supported on a knife-edge placed at the 40 cm graduation. It is found that the metre rule balances horizontally when a mass which has a weight of 0.45 N is suspended at the 15 cm graduation, as shown in the diagram. Calculate the moment, about the knife-edge in this balanced condition, of the force due to the mass of the rule.

You can see a copy of this question in this http://books.google.co.uk/books?id=...is supported on a knife edge placed"&f=false" or search a part of the text above.

I understand that Turning moment = Force x Distance from the pivot this means...

Distance = 40 - 15 = 25cm
Force = 0.45 N

This means 25 cm x 0.45N = 11.25N?

Thanks...
 
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Hi Googl! :smile:
Googl said:
Distance = 40 - 15 = 25cm
Force = 0.45 N

This means 25 cm x 0.45N = 11.25N?

centimetres? :wink:
 
tiny-tim said:
Hi Googl! :smile:


centimetres? :wink:

Oops...

So the work out and formula are correct, just the units?
 
yup! :smile:
 
To avoid confusion when the result of one calculation is passed to another calculation, it is as well to work in standard units. To work in SI Units, your distance would be in m and the moment you are after would be:
0.25m x 0.45N = 0.1125Nm
Not a pretty looking number but it is 'portable'.
 
Thanks. How about the second part.

If the centre of gravity is at 52.5cm graduation, calculate the mass of the rule.
 
Just write out the clockwise and equate them to the anticlockwise moments, with an x for the one quantity you don't know. Then solve the simple equation for x.
 
Sorry, Ignore this. [Edited]
 
So you're say...

wxd = wxd
0.45N x 0.25m = 0.60m x w
w = 0.188 N

I now have the weight on one side and weight on the other, now I should use these weights to find the mass and combine the mass. I realize I am not taking into account the 52.5cm graduation.
 
  • #10
Some help here please.
 
  • #11
Googl said:
A metre rule is supported on a knife-edge placed at the 40 cm graduation. It is found that the metre rule balances horizontally when a mass which has a weight of 0.45 N is suspended at the 15 cm graduation, as shown in the diagram. Calculate the moment, about the knife-edge in this balanced condition, of the force due to the mass of the rule.
Googl said:
If the centre of gravity is at 52.5cm graduation, calculate the mass of the rule.
Googl said:
wxd = wxd
0.45N x 0.25m = 0.60m x w
w = 0.188 N

I now have the weight on one side and weight on the other, now I should use these weights to find the mass and combine the mass. I realize I am not taking into account the 52.5cm graduation.

where did your 0.60m distance come from? :confused:

you need to use the distance from the centre of mass to the pivot point

(when you take moments, all moments must be relative to the same pivot point)
 
  • #12
Write out a proper equation using moments about some suitable point (actually, any point will do). If you have been given the problem to solve then you must have a book or notes with the basics of Moments Problems in. It always boils down to an equation with just one unknown in it.
 
  • #13
Distance from turning point to G is: 0.125m (d2)
d1 is: 0.25m
W x d1 = F x d2
W = F x d2 / d1
Mass = F x d2 / gd1
Mass = 0.45N x 0.125m / 9.8ms-2 x 0.25m
Mass = 0.023kg

Am I correct?
 
  • #14
(just got up :zzz: …)
Googl said:
Distance from turning point to G is: 0.125m (d2)
d1 is: 0.25m
W x d1 = F x d2

(try using the X2 icon just above the Reply box :wink:)

isn't it W x d2 ? :redface:
 

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