# The Naked Scientists Forum

### Author Topic: How does the position of a pivot affect force?  (Read 4416 times)

#### Mike Flanagan

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##### How does the position of a pivot affect force?
« on: 13/05/2009 16:30:02 »
Mike Flanagan  asked the Naked Scientists:

Hi Chris,

I am having an argument with a puzzle publisher....please look at this picture. http://www.flantoons.co.uk/mike.jpg

I say you need a pull-up force of 6 to keep the bar level.....but the editor says you only need a pull-up force of 3...because it is 2 feet from the pivot.....who is right?

Thanks.

Mike Flanagan

What do you think?

#### dentstudent

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##### How does the position of a pivot affect force?
« Reply #1 on: 13/05/2009 16:43:29 »
If you consider the force and distance on the left hand side: distance (D) = 1; load (L) = 6; therefore  force (F) = 6.

On the right; D = 2; L = 6; F = 12

If an upwards load was given of -6 (so D = 2; L = -6 thus F = -12) and added to the current load then the net F on the right would be 0, tipping the scales.

If D = 2; L = -3, then  F = -6.  12 + (-6) = 6. Thus a load of -3 balances the scales.

Which means that the editor is right...

#### lyner

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##### How does the position of a pivot affect force?
« Reply #2 on: 13/05/2009 16:44:04 »
You've got an anticlockwise moment of 6X1=6  and a clockwise moment of 3X2 = 12.
You need to supply an anticlockwise moment of 6, to balance. As you are applying the force at a distance of 2, it has to be 3 (3X2 = 6).
OK?
I had already written this when the last post went up.
There are more ways of killing a cat. . .  but I do like moments.

#### dentstudent

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##### How does the position of a pivot affect force?
« Reply #3 on: 13/05/2009 17:00:53 »
SC - your clockwise moment needs alteration!

3X2=12? Dumbing down, indeed!

Maths FOG

#### Don_1

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##### How does the position of a pivot affect force?
« Reply #4 on: 13/05/2009 17:11:06 »
SC - your clockwise moment needs alteration!

3X2=12? Dumbing down, indeed!

Maths FOG

No FOG about it, it's inflation!!!

#### dentstudent

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##### How does the position of a pivot affect force?
« Reply #5 on: 13/05/2009 17:12:27 »

#### lyner

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##### How does the position of a pivot affect force?
« Reply #6 on: 13/05/2009 19:33:00 »
On balance, I got the right answer, though.
Sorry chaps - but you know what I mean. 6X2 = 12 - take away 6 gives 6 blah blah blah

#### syhprum

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##### How does the position of a pivot affect force?
« Reply #7 on: 13/05/2009 20:57:29 »
Due to the position of the pivot the rotational force on the right side is multiplied by two hence for balance a reduction of three units is required.
I seem two remember that a short time ago half the mass of the Earth got lost perhaps some slide rule maintenance is required
« Last Edit: 13/05/2009 21:01:31 by syhprum »

#### lyner

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##### How does the position of a pivot affect force?
« Reply #8 on: 14/05/2009 12:21:12 »
Et tu syphrum?

#### dentstudent

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##### How does the position of a pivot affect force?
« Reply #9 on: 14/05/2009 12:28:01 »
Et tu syphrum?

The Ides of May is tomorrow....or is that Ide?

#### lyner

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##### How does the position of a pivot affect force?
« Reply #10 on: 14/05/2009 14:39:49 »
I have no ide a.

#### LeeE

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• Posts: 3382
##### How does the position of a pivot affect force?
« Reply #11 on: 15/05/2009 02:19:25 »
Mike Flanagan  asked the Naked Scientists:

Hi Chris,

I am having an argument with a puzzle publisher....please look at this picture. http://www.flantoons.co.uk/mike.jpg

I say you need a pull-up force of 6 to keep the bar level.....but the editor says you only need a pull-up force of 3...because it is 2 feet from the pivot.....who is right?

Thanks.

Mike Flanagan

What do you think?

A 'pull-up' force of six, applied in opposition to either of the down forces, will negate the corresponding down force.

With no 'pull-up' force, the down force of six on the longer arm will be equivalent to a down force of twelve on the shorter arm.  Thus, to balance the arm by applying a 'pull-up' force, you need to apply a 'pull-up' force of three to the down force of six on the longer arm, as indicated by the question mark.

In terms of applying a 'pull-up' force, the editor is correct.
« Last Edit: 15/05/2009 02:21:25 by LeeE »

#### The Naked Scientists Forum

##### How does the position of a pivot affect force?
« Reply #11 on: 15/05/2009 02:19:25 »