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# Homework help on moments Watch

1. I am very stuck on this question, the textbook give the answer but I have no idea how to get to that answer. The question is...

A metre rule, pivoted at its centre of mass, supports a 3.0 N weight at its 5.0 cm mark, a 2.0 weight at its 25cm mark and a weight W at its 80cm mark. What is the weight of W?

the textbook gives W as 6.2N. How does it get the answer and on which side if the pivot do the weights lie.

2. (Original post by Sani-Insanity)
I am very stuck on this question, the textbook give the answer but I have no idea how to get to that answer. The question is...

A metre rule, pivoted at its centre of mass, supports a 3.0 N weight at its 5.0 cm mark, a 2.0 weight at its 25cm mark and a weight W at its 80cm mark. What is the weight of W?

the textbook gives W as 6.2N. How does it get the answer and on which side if the pivot do the weights lie.

Taking moments about the centre of mass (ie the pivot point) (taking the cloackwise direction as positive):

(3x-0.45)+(2x-0.25)+(wx0.30)=0 , because it isn't moving =0
w=6.2 to 2dp.

Draw a diagram of the ruler and mark the weights at distances from the pivot point (the pivot point is halfway along ruler, which is where it's centre of mass is).

EDIT: I tried drawing a diagram but it didn't come out well when I submitted the post.
3. (Original post by mathperson)
Taking moments about the centre of mass (ie the pivot point) (taking the cloackwise direction as positive):

(3x-0.45)+(2x-0.25)+(wx0.30)=0 , because it isn't moving =0
w=6.2 to 2dp.

Draw a diagram of the ruler and mark the weights at distances from the pivot point (the pivot point is halfway along ruler, which is where it's centre of mass is).

EDIT: I tried drawing a diagram but it didn't come out well when I submitted the post.
Where did -0.45, -0.25 and 0.3 come from?
4. (Original post by Sani-Insanity)
Where did -0.45, -0.25 and 0.3 come from?
sorry, it should have been -3, -2 and +W, the distances should all be positive. The -ve sign described in the previous sentence comes from the fact that you need to take one positive direction, and one negative, due to the ability of the ruler to spin in either direction.
The distances 0.45, 0.25 and 0.30 are the distances that the weights are from the pivot point (the centre of the metre long ruler).

Does that help?
5. Thanks. It does help. Takes a lot of processing though. Does the distance in the moments=force x distance always mean the distance FROM the pivot point/ centre of mass.

Btw what doe a horizontal knife edge mean.

If you cbb to answer these extra questions thanks anyways for your time. Much appreciated.
6. (Original post by Sani-Insanity)
Thanks. It does help. Takes a lot of processing though. Does the distance in the moments=force x distance always mean the distance FROM the pivot point/ centre of mass.

Btw what doe a horizontal knife edge mean.

If you cbb to answer these extra questions thanks anyways for your time. Much appreciated.
it is best to quote me if you would like a response

Yes, the moment about a pivot point is equal to the force x distance from pivot point.

I'm not sure what that means, what context is it used in?
I would imagine that it means the contact surface is very small, but you will have to tell me the context.

7. (Original post by mathperson)
it is best to quote me if you would like a response

Yes, the moment about a pivot point is equal to the force x distance from pivot point.

I'm not sure what that means, what context is it used in?
I would imagine that it means the contact surface is very small, but you will have to tell me the context.

I'll just right the question:

A uniform metre rule supports a 4.5N wight at its 100mm mark. The rule is balanced horizontally on a horizontal knife edge at its 340mm mark. Sketch and calculate the weight of the rule.

Btw a rule is a ruler...right? sounds like a very stupid question for me to ask.
8. (Original post by Sani-Insanity)
I'll just right the question:

A uniform metre rule supports a 4.5N wight at its 100mm mark. The rule is balanced horizontally on a horizontal knife edge at its 340mm mark. Sketch and calculate the weight of the rule.

Btw a rule is a ruler...right? sounds like a very stupid question for me to ask.
yes a rule is a ruler.

A horizontal knife edge here is used to indicate the fact that there is no contact area between the rule and the pivot point.

so do the calculation, taking anticlockwise as -ve (or +ve depending on your preferance). I'll check later if you wish, but I must cook now.
9. I suck at physics. Dw there is nothing to check as I can't get an answer
10. take a picture of the sheet and upload it on imageshack and ill see :P
11. Would like to see the question before answering as well
12. (Original post by markrush)
Would like to see the question before answering as well

(Original post by blueray)
take a picture of the sheet and upload it on imageshack and ill see :P
I already wrote the question word for word from the book. The book provides no picture, you're supposed to come up with that yourself. I shall repost the question though.

A uniform metre rule supports a 4.5N wight at its 100mm mark. The rule is balanced horizontally on a horizontal knife edge at its 340mm mark. Sketch and calculate the weight of the rule.

13. Here's a diagram of the question as posted. Hope it helps.
Numbers on top are meter rule positions.
Numbers under are distances apart.
All in cm.
14. Okay nice and simple (misread the question)

The equation: m=fd

I am not being mean but people will learn a lot better talking things through than being told
15. (Original post by Stonebridge)
Here's a diagram of the question as posted. Hope it helps.
Numbers on top are meter rule positions.
Numbers under are distances apart.
All in cm.
amazing diagram really cleared up the first question. would it be possible to create one for the second question
16. (Original post by markrush)
Okay nice and simple (misread the question)

The equation: m=fd

I am not being mean but people will learn a lot better talking things through than being told
Working in metres i converted both 100 mm and 340mm to 0.1 and 0.34. I have the pivot on the 0.34 mark and the 4.5N wight on the 0.1 mark of the ruler. Unsure of what to do now.
17. (Original post by Sani-Insanity)
Working in metres i converted both 100 mm and 340mm to 0.1 and 0.34. I have the pivot on the 0.34 mark and the 4.5N wight on the 0.1 mark of the ruler. Unsure of what to do now.
what question are you working on now?
18. the one with the horizontal knife edge and ruler
19. (Original post by Sani-Insanity)
the one with the horizontal knife edge and ruler
the one with the horizontal knife edge and ruler
20. Here's a diagram for the other question.
I'll leave it to you to do the calculations.

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