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Unit, Dimensions and measurements > Physical-Quantity

Physical quantity

• A quantity that can be measured and by which various physical happenings can be explained and expressed in the form of laws is called a physical quantity. For example length, mass, time, force, etc.
• Measurement is necessary to determine the magnitude of a physical quantity, to compare two similar physical quantities and to prove physical laws or equations
• A physical quantity is represented completely by its magnitude and unit. For example, 10 metre means a length which is ten times the unit of length. Here 10 represents the numerical value of the given quantity and meter represents the unit of quantity under consideration.
  • Physical quantity (Q) = Magnitude x Unit = n x u
  • i.e. n u = constant, or n1u1 = constant ; ∴ n α 1/u

Types of physical quantities

• Ratio (numerical value only): When a physical quantity is the ratio of two similar quantities, it has no unit.
• Scalar (magnitude only): These quantities do not have any direction e g. Length, time, work, energy, etc. The magnitude of a physical quantity can be negative. In that case, a negative sign indicates that the numerical value of the quantity under consideration is negative. It does not specify the direction.
• Vector (magnitude and direction): These quantities have magnitude and direction both and can be added or subtracted with the help of laws of vector algebra e.g displacement, velocity, acceleration, force, etc.
• Scalar (magnitude only) : These quantities do not have any direction e g. Length, time, work, energy, etc. Magnitude of a physical quantity can be negative. In that case negative sign indicates that the numerical value of the quantity under consideration is negative. It does not specify the direction.