When is a quantity considered scalar?

A quantity is considered scalar when it has magnitude only and no direction.

In physics, quantities are often categorised into two types: scalar and vector. Scalar quantities are those that are fully described by a magnitude alone. In other words, they are quantities that have size or amount but no direction. This is in contrast to vector quantities which have both magnitude and direction.

Examples of scalar quantities include time, speed, mass, temperature, distance, and energy. These quantities can be described with just a single value, expressing a certain amount or size. For instance, when we say a car is moving at 60 kilometres per hour, we are describing its speed, a scalar quantity. We are not concerned with the direction in which the car is moving. Similarly, when we say an object has a mass of 10 kilograms, we are describing a scalar quantity. The mass is the same regardless of where the object is or in what direction it is moving.

It's important to note that scalar quantities can be added, subtracted, multiplied, and divided just like ordinary numbers. They obey the usual laws of arithmetic. For example, if you travel 5 kilometres north and then 3 kilometres south, your total distance travelled is 8 kilometres. This is because distance is a scalar quantity and you simply add the magnitudes together.

In summary, a scalar quantity in physics is a one-dimensional measurement of a quantity, like a temperature, or speed. Scalar quantities are the simplest physical quantities and they do not involve direction. Understanding scalar quantities is fundamental to physics, as it is the foundation upon which vector quantities, which are more complex, are built.