Math Is Fun Forum

Au101

Member

Registered: 2010-12-01

Posts: 353

Moments

Hi guys, I'm really stuck on this tricky moments question:

Figure 2 is attached.

Thanks very much .

Last edited by Au101 (2011-03-15 09:18:04)

Bob

Administrator

Registered: 2010-06-20

Posts: 10,583

Re: Moments

hi Au101,

The moment of a force is (the magnitude in Newtons) times (the perpendicular distance from the point to the force) ...{that you are taking moments about}

Another way to think about it is "it's the turning effect of a force".

For the first part the moments are about A.  Two forces act through A so the perpendicular distance for those two is zero => no turning effect.

The two remaining act clockwise and anticlockwise so subtract the smaller from the larger.

= 3N x 2m - 2N x 2m = 2 Nm (Newton-metre) in an anticlockwise direction.

For part two the moments are about D, so once again two forces have zero effect.

You are left with

3 x 2 + 5 x 2 - X times √2 (distance from D to the centre of the square)

= 8 x 2 - √2.X = 0

so X = 16/√2 = 16√2 / 2 (multiplying top and bottom by root 2) = 8√2 N

Bob

Last edited by Bob (2011-03-15 11:29:54)

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

Au101

Member

Registered: 2010-12-01

Posts: 353

Re: Moments

Aha, thank you very much yet again Bob. Also, I like your new quote

Bob

Administrator

Registered: 2010-06-20

Posts: 10,583

Re: Moments

hi Au101,

You are welcome.

My daughter has a book full of quotes listed alphabetically by topic.  So I had lots that I liked.  This was one I particularly liked for this forum.  Some were brilliant but not suitable for family viewing!

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob