rotational dynamics - What is a moment?

As a 8th grader I am still a newbie at science and I have problems understanding the forces that I can not imagine and one of these "forces" is the moment.

We are told that,

$$\text{moment}= Nm$$ but how can a certain force (i.e. $N$) be applied when there is $m$? How does it depend on the length of the considered object?

And how can 1 moment be defined on the basis of that representation?

And what would be the effect if the moment value is increased by the newton part as opposed to the effect when the value is increased by the length part.

Or is the result the same in both cases?

Answer

The concept of a moment (or force moment) is that of a force at a distance. It not only measures how strong the force is but also how far away it is applied (for rotational purposes).

Consider s see-saw with a bear and a skunk.

In order to quantify the idea of "balance" you need a description of the weight of each object as well as how far away does this weight apply relative to the fulcrum (red triangle).

You equate the moments

$$\mbox{moment of bear} = \mbox{moment of skunk}$$ $$x_B\, W_B = x_K\, W_K$$

_{where $x_B$ and $x_K$ are the distances (of the bear and the skunk respectively), and $W_B$ and $W_K$ the weights (of the bear and the skunk respectively).}

The units are $\mbox{[length]} \times \mbox{[distance]}$ for moments. In the SI sytem that is $\rm N\,m$ (Newton-meters) and in the customary units $\rm lbs\;ft$ (Foot-pounds).