What is Velocity? 

Velocity is a vector quantity and used to determine the rate of change of an object’s position. As it is a rate of change of an object position if you move 10 metres forward and then 10 metres backwards, your velocity is zero.

Even if you move between these points rapidly, your velocity is zero because it is determined by the rate of change of an object’s position; you started and finished in the same position. 

To maximise velocity, the object or person must displace themselves, the largest distance from the original starting position. 

Velocity is a vector quantity, which means it has a directional function. When determining the velocity of an object, you must also determine its direction.

The velocity is always different in race cars
A racing cars velocity is always different as they are constantly turning

For example, it would be incorrect to state an object’s velocity as just 15 m/s. For it to be correct, a direction must accompany it, therefore, 15 m/s west.

A second example is racing cars turning on a track. If they maintain the same speed whilst going round a bend, like in NASCAR, their velocity is constantly changing because so is their direction.

Another example is terminal velocity. Terminal velocity is the maximum velocity attainable for an object falling through a fluid (i.e. air or water). Terminal velocity occurs when the drag forces experienced on the falling body are equal to the acceleration forces on the body due to gravity.

At this point, the body is no longer accelerating and has reached terminal velocity. As discussed above, velocity must have a direction, and the direction for terminal velocity is down (towards the centre of the earth) as it is related to gravity.

However, at university and good practice, velocity is determined using the i,j,k vectors which have a direction in their respective planes (x,y,z). For example, an object’s velocity may be equal to 5i + 3j + 2k. 

What is Speed?

Speed is a scalar quantity. Scalar quantities may be described by a single number with its accompanying units. Unlike velocity (a vector quantity), it does not require a direction. A scalar is considered multi-directional. 

Speed is defined as the rate at which an object covers distance. A fast-moving object will cover a large distance over a short period, giving it a high speed. 

A speedometer shows the speed of an object, not the velocity as it does not determine direction.
A speedometer shows the speed on an object

Looking back at our first example, if you move 10 metres forward and then 10 metres backwards, you have moved 20 metres in total. If this is completed in 5 seconds, your speed will be 20 metres / 5 seconds = 4 m/s.

Unlike velocity, the direction of the movement is not considered in the calculation.

Another example is the speed of sound. The speed of sound is approximately 343 m/s. But what direction does it travel? Sound doesn’t have one particular direction it travels multi-directional, meaning it is a scalar product. Hence it is called the speed of sound, not the velocity of sound.

The Equations For Velocity and Speed

The best way to tell the difference between velocity and speed is by comparing their equations. 

The formula for speed is the distance (d) travelled divided by the time taken to travel that distance(t):

s= d/Δt

The velocity formula is the displacement (Δs) divided by the time to displace said distance(Δt). Velocity formula: 

v = Δs/Δt

For the everyday person, the difference between speed and velocity isn’t a big deal. However, if you are an engineer or physicist, it is important, you know the difference between the two.

Velocity and speed are the same when the direction is in a straight line. If the values between speed and velocity are different, it indicates the objects travel direction was not straight.

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