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What is dimension of a physical quantity? 1. In all the systems of units, the derived units of all the physical quantities can be expressed in terms of the  fundamental units  of mass, length and time, raised to some power. The powers to witch fundamentals units must be raised in order to express a physical quantity, are called its dimensions. To make it clear, consider the physical quantity density which is defined as mass per unit volume. Hence,  density=mass/volume=M/L 3 =[M1L 3 T 0 ]. Dimensions of a Physical Quantity 2. a) Above expression shows that dimensions of a physical quantity (density) are 1 in mass, -3 in lengths and 0 in time. c) Dimensional formula for density is [ML -3 ] or [ Ml -3 T 0 ]. d) Dimensional equation for density is [d] =[ML -3 T 0 ]. Dimensional formula of some physical quantities in mechanics and their units in different systems Physical Quantity Formula Dimensional Formulae S.I Unit Area Length × breadth [L 2 ] m 2 Vol...