Differences Between Speed and Velocity

Speed and velocity are two terms most people use interchangeably when they want to refer to a moving object. However, speed and velocity are two different concepts in kinematic physics. In this article, we will explore the concepts behind speed and velocity and explain how they differ from one another.

What’s Speed and Velocity?

What’s Speed and Velocity?
What’s Speed and Velocity, Image Courtesy of AdobeStock

Speed is how fast an object is moving; it is calculated as the ratio of distance traveled by an object per unit time, it is thus a scalar quantity. In contrast, velocity is the rate at which an object changes position in a certain direction, it is calculated as the ratio of displacement of an object per unit time in a certain direction. Velocity deals with direction and thus it is a vector quantity, while speed does not (it’s the magnitude of velocity).

Speed VS Velocity

Speed VS Velocity
Speed VS Velocity – fast train, Image Courtesy of AdobeStock

Although velocity includes direction, it’s still possible for two objects to have the same speed and velocities. For example, if you traveled by bus to visit a friend, you might go 60 Km/h (kilometers per hour) north on the way there and 60 km/h south on the way back. Your speed and velocity are the same (speed = 60 Km/hr, velocity = 60 Km/hr) while you’re going to visit your friend but as soon as you change direction and you’re on the way back to south, speed remains the same (60 Km/hr) but the velocity changes because your direction of motion has changed from north to south. In fact, velocity goes to zero in this case, as the total displacement becomes zero (you returned to where you have started from).

Another key difference between speed and velocity is the way the ‘average’ is calculated. Your average speed depends on the total distance you have traveled. The average velocity depends on the total distance you are from where you started. Going north, your average speed would be 60 Km/h, and your average velocity would be 60 Km/h north. After the round-trip ride, your average traveling speed would still be 60 Km/h. Your average velocity, however, would be 0 Km/h because you ended up exactly where you started.

As velocity is a vector quantity (have both magnitude and direction), it’s magnitude can be negative. However, speed is always positive, it can’t be negative (it deals with magnitude only not direction).

We hope that this simple explanation of the concepts behind speed and velocity and their differences have helped you.

Aimal Khan is the founder & CEO of Engineering Passion. He is an engineer and has obtained his bachelor's degree in energy engineering from Kandahar University.