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This principle is essential for example when dealing with a cable which is used to support a crane.

The internal deformation inside the cable and the strains in the crane structure can be neglected
and only the external forces that act on the cable and support are taken into account.

Figure 1.4: Differential between Soft Body and Rigid Body

1.3 QUANTITIES AND UNIT

The four basic quantities in mechanics are mass, forces, length and time, they are not all
independent from one another. These quantities are all related to one another by Newton’s Second

Law. The four fundamental quantities and their unit are shown in table below.

Table 1.1: Fundamental Quantities and Units

Quantity Dimensional Unit Symbol of Unit
Symbol

Mass m Kilogram kg

Forces F Newton N
Length l Meter m

Time t Second s

1.3.1 MASS

Mass is a measure of the inertia of body or it is a measure of the amount of matter in an object.

Mass is usually measured in SI unit for mass kilogram (kg) and the dimensional symbol of mass

is ‘m’

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