# Very simple moments question

## Main Question or Discussion Point

I can't put it more simply than this:

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chroot
Staff Emeritus
Gold Member
Well, this doesn't make much sense to me. What's the red thing? What's the blue thing? What's the pivot? It looks to me like a blue brick sitting on a table. I don't get it.

- Warren

Hi everyone,

If the first is equilibrium then the second is not.

In the first picture the centre of the gravity of the blue object is on the right of the pivot, let’s say that the distance between the pivot and the centre of the gravity of the blue object is D.

Similar the distance between the pivot and the centre of the gravity of the red object is d then if we want to have equilibrium the moments of the two object must be equal so we have: md=MD (1) (M: the mass of the blue object and m: the mass of the red object).

With similar thoughts in the second picture (we have supposed that there is equilibrium in the 2nd picture) you end up to the equation md’=MD (2) (d’: is the new distance between the pivot and the centre of the gravity of the red object).

If we had equilibrium in the 2nd picture then from (1) and (2) we have: d=d’ which is atop. So in the 2nd picture we do not have equilibrium.

Originally posted by chroot
Well, this doesn't make much sense to me. What's the red thing? What's the blue thing? What's the pivot? It looks to me like a blue brick sitting on a table. I don't get it.

- Warren
It's an object with an extension stuck on to it. Like, for instance, a hatchback with the trunk (or boot) open/shut. It's on a balance.

The centre of mass of the single blue+red object doesn't move, but the object does change shape - the red section moves. The question is will this change the moment?

Seemingly yes.

chroot
Staff Emeritus
Gold Member
Originally posted by Lord Ping
The centre of mass of the single blue+red object doesn't move
Bull**** -- of course it does.

- Warren

It moves down slightly, but that doesn't matter... does it move horizontally too?

chroot
Staff Emeritus
Gold Member
Of course.

- Warren

Integral
Staff Emeritus
Gold Member
Pehaps the ending of this little video is what this is all about.

I love that video!

i dont think that is compleatly correct