Speed, velocity and acceleration are three important concepts in kinematics, and closely associated with each other. Considered together, the three concepts offer an almost complete insight of motion. Speed is related to distance and is known to be a scalar quantity. You must understand that speed differs from velocity, in which case the speed is considered to be in a given direction. Acceleration, according to Newton’s second law, is the net result of the forces acting on a particular object. Let’s try to understand in detail the difference between the three concepts with this article.

## Definitions

**Speed** can be defined as magnitude of the velocity of an object, or rate of change of position of an object. Speed is a scalar quantity. An object’s average speed can be calculated when distance (d) covered by it is divided by the time (t) consumed by it. The physical units of speed and velocity are the same, though the element of direction is lacking when it comes speed.

**Velocity** takes into consideration both the components of magnitude and direction – the speed of an object in a given direction is its velocity. Thus, velocity is a vector quantity. It cannot be said that the car is moving at a velocity of 45 km/hr. You need to specify the direction as well. For an example, the car is moving at a velocity of 45 km/hr to the north.

**Acceleration** is rate of change of velocity. Therefore, it is also a vector quantity. The SI unit of acceleration is meter/second². Accelerating an object signifies changing its velocity i.e. either changing the direction or speed of the object in a given time period. Acceleration of an object may vary between positive and negative.

If the sign of acceleration – positive or negative – is the same as that of velocity, the object’s speed increases. In case the acceleration sign is positive and velocity is negative, or vice versa, the object decelerates. Force needs to be applied to accelerate the object.

## Comparison

Speed and velocity are closely related to each other, with the only difference being the absence of the direction element in case of speed. If someone drives at 40 km/hr, it is his speed. The velocity is not known in this case, until the direction is identified. When velocity is changed somehow, it is termed as acceleration. Acceleration and velocity in the same direction speed up the object. In the opposite case, the object slows down.

## Comparison Chart

Speed | Velocity | Acceleration |

Speed is a scalar quantity. | Velocity is a vector quantity. | Acceleration is a vector quantity. |

Units of average speed = m/s (meter/second) | Units of average velocity = m/s | Units of average acceleration = m/s^{2 } |

Average speed = distance travelled/time elapsed = Δx/Δt | Average velocity = change in displacement/time elapsed = Δs/Δt | Average acceleration = change in velocity/time elapsed Δv/Δt |

## Video

This video beautifully and simply explains the terms speed, velocity and acceleration.