What a moment is
The quantity that brings the force and the perpendicular distance together has a name: the moment of a force.
Moment of a force
The product of the magnitude of a force and the perpendicular distance from an axis to the line of action of that force is called the moment of the force about that axis.
moment = force × perpendicular distance
The perpendicular distance is measured from the axis to the line of action of the force.
The unit of a moment
Force is measured in newtons (N) and distance in metres (m). Because the two are multiplied, the unit of a moment is the product of both.
N m
The newton metre — read aloud as "newton metre".
N m is not N/m
What sits between the two units is a multiplication, not a division. Write it N m, with a space; written N/m it is an entirely different quantity.
The last column of table 11.1
The column left empty in activity 1 can be filled in now. At each of A, B, C and D, multiply the force by its perpendicular distance.
Observation
At all four points the product comes out roughly the same. The force changed and the distance changed, but the product of the two did not.
So what starting the door turning actually requires is not a particular force — it is a particular moment.
A force alone tells you nothing
"It takes 8 N to open the door" is an unfinished statement — without saying where, it means nothing. The moment is the complete answer, because it carries both the force and the place inside itself.