How to Calculate Average Speed in Physics

Average speed is the total distance travelled divided by the total time taken, providing a measure of the overall rate of motion over a given period. To calculate average speed in physics, divide the total distance travelled by an object by the total time it takes to cover that distance. The average speed formula is:

Average Speed = Total Distance / Total Time

How to Calculate Average Speed in Physics
How to Calculate Average Speed in Physics

Tips and Tricks for Efficient Calculation

While the formula is straightforward, a few tips can enhance your efficiency:

  1. Break Down Complex Problem: Split complex problems into segments, calculate average speeds for each, and then combine them. You can start by drafting your data before you solve the problem.
  2. Consistent Units: Ensure consistency in units for distance and time to avoid errors.
  3. Verify Speed-Time Combinations: Double-check that speed-time combinations align with the problem’s chronological order.

These tips will streamline your average speed calculations for accuracy.

What is Speed?

Definition: Speed is the rate of change of distance with time. It is a scalar quantity because it has only magnitude but no direction. The s.i unit for speed is meter per second (m/s) or kilometre per hour (km/h). The formula for calculating speed is s = d/t, where s is a symbol for speed, d for distance, and t for time. Here is the formula:

Speed (s) = Distance (d) / Time (t)

As we can see from the above formula, speed is the ratio of distance to time. For us to understand the concept of speed, we need to understand the definitions of distance and displacement. Distance is the measure of separation between two points. While we can define displacement as the measure of separation between two points in a specified direction.

We can clearly see from the two definitions above, that the one distance is a scalar because it doesn’t have direction. While displacement is in a specified direction which makes it a vector quantity. For example, when you leave your house for school, your road may not be straight. You may encounter many junctions along the way. Hence, since the movement doesn’t have to be specific. By using the time it takes you to move from your house to your school, we will be able to find your speed. Speed doesn’t have to be in a specific direction, while velocity is in a specific direction. We also need to understand that speed is a measure of how fast a body is going.

What is Average Speed?

Definition: An average speed is the ratio of the total distance travelled to the total time taken. We use A.S as the symbol for average speed. We also measure it in meters per second (m/s) and it is a scalar quantity. The formula for calculating the average speed is A.S = T.D / T.T where T.D is the total distance travelled, and T.T is the total time taken. Here is the formula:

Average Speed (A.S) = Total Distance Traveled (T.D) / Total Time Taken (T.T)

When the distances covered with time over a particular journey are not steady, possibly due to traffic conditions, then we can say that the speed is a non-uniform speed. Under such a situation, the total distance covered over a time period is referred to as the average speed.

Definition of Actual Speed: This is the instantaneous speed of an object at a particular measured distance over a short interval of time.

Problems on How to Calculate Average Speed in Physics

Here are a few solved problems that will help you to understand how to calculate average speed in physics:

Problem 1

A student walks a distance of 3 kilometres in 20 minutes. Calculate his average speed.

Answer

The average speed of the boy is 2.5 meters per second (m/s)

Explanation

Data: The information from the question

The total distance covered by student, D = 3 km = 3 x 1000 m = 3000 m

Time taken, T = 20 minutes = 20 x 60 s = 120 s

Unknown: What we need to find

Average Speed, A.S = ?

Formula: The equation that will help us solve the problem

Average Speed (A.S) = Total Distance Traveled (T.D) / Total Time Taken (T.T)

Which implies that A.S = T.D / T.T

Solution

We will now insert our data into the formula

A.S = T.D / T.T = 3000 / 120 = 2.5 m/s

Therefore, the average speed of the student is 2.5 meters per second.

Problem 2

A driver travelling at a speed of 115 kilometres per hour received a text message on his mobile phone. How far is he, in kilometres, 20 seconds later from when he received the text?

Answer

The driver covers 0.639 kilometres in 20 seconds.

Explanation

Speed = 115 km/hr

Time = 20 seconds = (20 / 3600) hr

Distance = speed x time = 115 x (20 / 3600) = 2300 / 3600 = 0.639 km

Therefore, the driver covers a distance of 0.639 kilometres in 20 seconds.

Problem 3

A car travels along a straight line covering a distance of 20 meters in 2 seconds. What is the speed of the car?

Answer

The speed of the car is 10 meters per second

Explanation

Speed = Distance / Time = 20 / 2 = 10 m/s

Therefore, the speed of the car is 10 meters per second.

Problem 4

A car travelled a distance of 4 kilometres in 40 seconds. Find the speed in meters per second (m/s).

Answer

The speed of the car is 100 meters per second

Explanation

Distance = 4 km = 4 x 1000 m = 4000 m

Time = 40 s

Speed = Distance / Time = 4000 / 40 = 100 m/s

Therefore, the speed of the car is 100 meters per second

Problem 5

Consider a train that travels at 20 km/h for the first three hours and accelerates to 30 km/h for the next four hours. To find the average speed, we apply the formula:

Average Speed = [(20×3)+(30×4)] / (3+4)​

Following the steps, we get an average speed of 25.7 km/h. This example illustrates how the formula accommodates varying speeds seamlessly.

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