Mass is a fundamental quantity; the other above-mentioned mechanical quantities are derived from it. Its unit, 1 kilogram, is a basic SI (Système Internationale) unit. Mass means ‘quantity of matter’. This description is more or less a tautology; it shows the difficulty in defining the fundamental. Our perception of mass is that of weight, or of an object's opposition to an attempt to move it, which means applying force. In physics, weight in fact is a force, defined as the product of mass and acceleration: F = m.a. Its unit is 1 newton (N) or 1 kg m per s2. The gravitational pull is the same in most places on Earth, and gives all objects an acceleration of 9.81 m per s−2. Thus weight is directly proportional to mass, and the two quantities are interchangeable. The kilogram is therefore used as a unit of weight, instead of newton, which is the correct unit. The relationship between mass and weight can be readily demonstrated on weighing scales: 1 kg of mass placed on one side of the scale produces a force (weight) of 9.81 newtons. Consequently, when training operating department assistants, to exert a force of 20 to 40 newtons (for the application of cricoid pressure), the weight they should be aiming for on the scales is 2 to 4 kg.