Average Speed

Introduction

Speed is the total distance travelled by an object in unit time, whereas velocity is the total displacement travelled by the object in unit time. You must already know about speed and velocity and the different types of properties that both of them possess. 

Now, we are going to learn about average speed and average velocity. Are they the same? What factors affect them? What is the formula to find the average speed and average velocity? We will find the solution to all these questions.

Definition of average speed

The total distance travelled with respect to the total time taken is known as average speed. The average speed of a moving object tells you at how much average pace you are going to cover a certain distance. 

For example, if you have travelled 100 KM in 2 hours then simply average speed tells you that you have covered 50 KM in an hour as your average speed. The SI unit of Average speed is m/sec.

Formula of average speed

We can find the average speed of a moving object by a simple formula.

S = d/t  where S denotes average speed, d is the total distance travelled and t stands for total time taken.

So clearly the unit of average speed is m/sec.

Let’s solve a problem to have a better understanding of this formula.

Problem

A motorcycle rider travels a distance d at 50KM/hr then he travels at 60KM/hr for a distance 2d, so what was the average speed at which he was travelling?

Let the first time as T₁ = d/50

Let the second time as T₂= 2d/60

So by the formula, avg speed = total distance/ total time taken

= d+2dd50 + 2d60

=900/16

= 56.25 KM/hr

Is the average speed scalar or vector?

Average speed does not have any direction and is a scalar quantity. Moreover, it can only be positive and zero but not negative.

Definition of average velocity

The total displacement covered with respect to the time taken is known as average velocity. It can also be defined as the ratio of displacement to time. Similar to average speed, the SI unit of average velocity is also m/sec.

But unlike average speed, average velocity does have a direction and it always is in the direction of displacement. Even if speed and magnitude are constantly changing, the direction of average velocity would still be the same as the direction of displacement.

Formula of average velocity

We can find the average velocity of a moving object by the given formula:

V = x/t, where x stands for displacement and t stands for the time taken.

Or

V= Xf – XiTf – Ti, 

where Xi = initial displacement, Xf = final displacement

 Ti = initial time, Tf = final time

As the unit of displacement is metre and unit of time is seconds, thus the SI unit of average velocity is m/s.

Let’s understand this by solving another problem.

Problem

Find the average velocity at a specific time interval of a car if it moves 7 metres in 4 seconds and 18 metres in 6 seconds along the x-axis.

Initial distance covered by the car x₁= 7m

Final distance covered by the car x₂= 18m   Initial time t₁= 4 sec

Final time t₂= 6 sec

By the formula of average velocity = Total displacement/ total time taken=> x₂-x₁t₂-t₁

=> 11/2

=> 5.5 m/sec

Is average velocity scalar or vector?

As velocity includes both magnitude as well as direction, therefore average velocity is a vector quantity. Its direction is always in the direction of displacement. Moreover, average velocity can be positive, negative, or zero.

Comparison of average speed and average velocity

Similarities

The SI unit of both average speed and average velocity is the same, that is metres per second. The formula which is used is almost the same. In average velocity, you have to mention the direction so there has to be a specific sign with respect to the displacement.

Differences

Average speed is a scalar quantity while average velocity is a vector quantity. Being a scalar quantity, the final average speed is not affected by the direction. Contrary to that, average velocity is a vector quantity and its final outcome will depend on the direction of displacement.

While calculating average speed, distance is taken into consideration, while in the case of average velocity, we have to deal with displacement.

Conclusion

So this was all about average speed and average velocity. If you have read it from start to bottom, now, you must have surely understood the topic in a better way. We have tried to keep things simple so that you can understand it easily. 

Always stick to the basics and you will surely be able to hunt down the most complex problems. Don’t try to rush and take your time in understanding such topics. They might look the same but, in reality, they aren’t and that is why students sometimes make silly mistakes in calculating average speed instead of average velocity.