In physics, the words ‘speed’ and ‘velocity’ have different meanings, although they are closely related: velocity is an object’s speed in a particular stated direction. So, we could say that an aircraft has a speed of 200 m/s but a velocity of 200 m/s due north. We must give the direction of the velocity or the information is incomplete.
Velocity is an example of a vector quantity. Vectors have both magnitude (size) and direction. Another example of a vector is weight – your weight is a force that acts downwards, towards the centre of the Earth.
Speed is an example of a scalar quantity. Scalars only have magnitude. Temperature is an example of another scalar quantity
A scalar quantity is defined as the physical quantity with only magnitude and no direction. Such physical quantities can be described just by their numerical value without directions.
A vector quantity is defined as the physical quantity that has both directions as well as magnitude.
Speed - Time Graph
Just as we can represent the motion of a moving object by a distance–time graph, we can also represent it by a speed–time graph. A speed–time graph shows how the object’s speed changes as it moves.
Always check any graph by looking at the axes to see the labels. A speed–time graph has speed on the vertical axis and time on the horizontal axis.
This is a Speed and time graph which represents the amount of speed a bus travel in the given amount of time
Graphs of different shapes
Sloping upwards, so the speed increases and the train is accelerating
horizontal, so the speed is constant and the train is traveling at a steady speed
sloping downwards, so the speed decreases and the train is decelerating
horizontal, so the speed has decreased to zero and the train is stationary.