The result of a force acting at a distance.
The Principle of Moments
Apply a force at a distance and you have a turning effect which we call a moment. I show you how to calculate the moment when a force acts at an angle. Finally, for an object in rotational equilibrium the sum of the anti-clockwise and clockwise moments is zero which is the principle of moments.
Torque of a Couple
If you have two equally sized forces that acts towards each other but are offset then you have a resulting moment but no resulting force. Because there are two forces we call this the torque of a couple.
Density is a measure of the amount of 'stuff' per unit volume. But what happens when you add salt to an egg in water?