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Speed and Velocity are two frequently used derived quantities in Physics.
Length, Mass and Time – Each of these 3 physical quantities doesn’t depend on any other quantity for its measurement. So these 3 are called Fundamental quantities.
Other quantities like velocity, acceleration, and force all depend on more than one fundamental quantity for their measurements. These are called Derived Quantities. [ like velocity depends on length and time measurement ]
To understand speed and velocity, let’s first quickly discuss on 2 quantities: Displacement and Distance.
Displacement and Distance
Distance is the length actually traveled on a route while Displacement is the shortest distance between the initial point and final point of the traveled path.
So if anybody runs a completely circular route once then the Distance traveled is equal to the circumference of the circle i.e. 2.Pi.R where R is the radius of the circle (and Pi is 22/7). At the same time the Displacement = 0 because the final point is same as the initial point.
Similarly if the runner runs half of the circular track then the Distance traveled is Pi.R and the Displacement is 2R or Diameter of the circle.
Speed and Velocity
Now Speed is the distance traveled per unit time = (distance traveled)/(time taken)
And the velocity is displacement per unit time = (displacement)/(time taken)
Only when something moves in a straight line in a fixed direction then at any point in time distance and displacement become equal, henceforth in this case speed and velocity magnitude become equal.
One more thing, Speed doesn’t need mention of any direction of movement while Velocity needs a direction to be mentioned along with its magnitude. Velocity is a Vector quantity and Speed is a scalar quantity. Will discuss more on Vector in one of the next posts.