Speed and velocity are related to one another in the same way that distance and displacement are related to one another. Speed is a scalar quantity just like distance is a scalar quantity (scalar measurements do not have a direction). Velocity is a vector measurement just like displacement is a vector (vector measurements have a direction). We can calculate speed by using distance because they are both scalars. We can calculate velocity by using displacement because they are both vectors.
Speed is the rate at which an object is travelling a certain distance. Speed is really ‘how fast you are travelling.’ Speed includes two measurements: distance and time. You have probably seen speed limit signs posted on the side of the road. For example, a speed limit of 100 km/hr, means that you can travel a distance of 100 km in one hour. If you travel a distance that is greater than 100 km in one hour, then you are at risk of receiving a ticket from a police officer. Luckily, you car comes equipped with a speedometer to tell you how fast you are travelling.
Velocity is the rate at which an object changes its position. The difference between speed and velocity is that velocity is going to include a direction. It is not enough to say that your velocity is 100 km/hr, you must say that your velocity is 100 km/hr, east.
Since velocity is related to displacement, it is possible to have an average velocity of 0.0 even though you may have been moving. For example, if you were to to run from your front porch to your mailbox that’s near the road and then run back to your front porch, your displacement would be 0.0 m. Your starting position was your front porch and your ending position was also your front porch. Displacement only cares about your starting position and your ending position. Even though you may been running very quickly to and from your mailbox your average velocity was 0.0 m/s.
You can think of it this way, if you ran 10 metres to your mail box (we will call that the positive direction) and then 10 metres back to your porch (we will call that the negative direction since its in the opposite direction), what would you get if you add those two value together?
10 m + (-10 m) = 0 m.
So you didn’t actually have any displacement and so you didn’t have an overall velocity.
Calculating Speed and Velocity