Moment of force and the meter rule

[Googl]

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...

 

[tiny-tim]

Hi Googl!

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

This means 25 cm x 0.45N = 11.25N?

centimetres?

 

[Googl]

Hi Googl!


centimetres?

Oops...

So the work out and formula are correct, just the units?

 

[tiny-tim]

yup!

 

[sophiecentaur]

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'.

 

[Googl]

Thanks. How about the second part.

If the centre of gravity is at 52.5cm graduation, calculate the mass of the rule.

 

[sophiecentaur]

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.

 

[Googl]

Sorry, Ignore this. [Edited]

 

[Googl]

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.

 

[Googl]

Some help here please.

 

[tiny-tim]

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.

If the centre of gravity is at 52.5cm graduation, calculate the mass of the rule.

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?

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)

 

[sophiecentaur]

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.

 

[Googl]

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?

 

[tiny-tim]

(just got up :zzz: …)

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 )

isn't it W x d₂ ?