Scalars and vectors quantities
All physical quantities that we measure with instruments can be classified into two broad categories - scalar quantities and vector quantities. Some quantities may be completely specified by stating a single number, for example, the age of a person, the mass of an object, and volume of air or temperature of the room. In other words, they have a magnitude but no direction.
Measurable quantities that have magnitude only are called scalar quantities or simply scalars. On the other hand, some physical quantities are completely specified not by merely stating a number but a direction as well. Measurable quantities that have a magnitude as well as a direction are called vector quantities, or simply, vectors.
A few examples of scalar quantities are time, temperature, mass, distance, area, volume, speed, work, energy, power, charge, potential, etc. A few examples of vector quantities are displacement, velocity, acceleration, force, momentum, moment of force (or torque), electric field, magnetic field, etc.
The Parallelogram of Forces:
This is a rule used when adding together two vector quantities. It states that 'if two forces F, and F,, acting at a point, are represented to scale in both magnitude and direction by the two adjacent sides of a parallelogram, then the diagonal from their point of intersection to the opposite corner of the parallelogram represents the result-
The parallelogram rule is used to help navigation at sea. The direction and speed of the tide must be taken into account as the second vector quantity to be added to the direction and speed of the boat.
Non-uniform motion :
Bodies do not always move with uniform speed. A train starting from a station moves very slowly in the first second, i.e., it covers a very small distance during the first second and then speeds up covering a larger distance in the next second and so on.
In other words, the train does not cover equal distances in equal intervals of time. If a body covers unequal distances in equal intervals of time, its motion is said to be non-uniform.