what is mean by physical quantity?

Answer 1

A physical quantity is some aspect of the physical world that can be measured (that is, can have a number attached to it). It's something that has an "amount". For example, "mass" is a physical quantity, because it's something that can be measured in kilograms; and "speed" is a physical quantity, because it's something that can be measured in miles per hour.

A physical quantity has to have "units" (like, "pounds" or "inches" or "hours" or "degrees per second") attached to it. "37" is not a physical quantity; but "37 volts" is.

Also, the units generally need to have something to do with physics and the physical world. The words "5 dollars per hour" represent a quantity with units; but this is gnerally not considered to be a _physical_ quantity.