Homework help on moments

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.

Please answer as I am desperately trying not to fail Physics. :confused:

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.

(edited 13 years ago)

Reply 2

OP

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?

Reply 3

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?

Reply 4

OP

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.

Reply 5

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 :smile:

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 don't mind answering lol. :smile:

Reply 6

OP

Original post by mathperson

it is best to quote me if you would like a response :smile:

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 don't mind answering lol. :smile:

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.

Reply 7

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.

Reply 8

OP

I suck at physics. Dw there is nothing to check as I can't get an answer

Reply 9

User570431

15

take a picture of the sheet and upload it on imageshack and ill see :P

Reply 10

markrush

11

Would like to see the question before answering as well

Reply 11

OP

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.

Thanks for any help you can give.

Reply 12

Stonebridge

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.

(edited 13 years ago)

Reply 13

markrush

11

Okay nice and simple (misread the question)

The equation: m=fd

What are your thoughts about how to use it?

I am not being mean but people will learn a lot better talking things through than being told

Reply 14

OP

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

Reply 15

OP

Original post by markrush

Okay nice and simple (misread the question)

The equation: m=fd

What are your thoughts about how to use it?

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.

Reply 16