Question
A student walks a distance of 3 km in 20 minutes. Calculate his average speed.
Quick Answer
Average speed = Distance/Time = 3km / 20minutes = (3 x 1000m) / (20 x 60s) = 2.5 m/s
Understanding the question with detailed explanations
A student walks a distance of 3 kilometres in a total time of 20 minutes. The question is asking us to determine his average speed during this walk. Average speed is simply the total distance covered divided by the total time taken. It gives us a measure of how fast the student was moving over the entire journey, without worrying about whether he sped up or slowed down at different moments. In other words, we are not looking at his instantaneous speed at any point, but rather at the single uniform speed that would have gotten him from start to finish in exactly the same time.
This type of question is common in introductory physics because it lays the foundation for understanding motion. Distance and time are two of the simplest quantities we can measure, and speed connects them in a straightforward way. The situation here is uncomplicated: the student walked in a straight path, with no mention of obstacles, detours, or rest stops. Thus, we can assume a smooth and continuous motion where the average speed will truly capture the entire story of the walk. The main challenge is converting all the units correctly to make sure the final answer is expressed in a standard and useful form.
A glimpse of the final answer
The student’s average speed turns out to be 9 km/h or equivalently 2.5 m/s. These two forms are both correct; the difference lies in the unit system being used. In real life, when we talk about walking or running, kilometres per hour is often more convenient, since cars, treadmills, and everyday activities are usually measured this way. However, in physics classrooms and exams, metres per second is the standard SI unit, so it is important to express the answer in both.
We can also quickly check if this answer makes sense. Normal human walking speed is typically around 4 to 5 km/h. Here, the student’s speed is 9 km/h, which is significantly faster. This suggests that instead of a leisurely stroll, the student was likely walking briskly, maybe even close to jogging pace. The order of magnitude is correct, clearly not in tens or hundreds of kilometres per hour, which would be unrealistic for a human walking on foot. This gives us confidence that our calculation is reasonable.
Data
Let’s carefully write down the information given in the problem. The distance covered by the student is 3 kilometres. The time taken is 20 minutes. These are the two main quantities provided, and they are sufficient to calculate average speed. However, to work neatly in physics, we usually convert everything into SI units first. That means distance should be expressed in metres, and time should be expressed in seconds.
So the conversions are as follows: 1 kilometre equals 1000 metres, so 3 km = 3000 m. Similarly, 1 minute equals 60 seconds, so 20 minutes = 20 × 60 = 1200 seconds. Thus, our given data in SI units are: distance = 3000 m, time = 1200 s. The unknown we are solving for is average speed, measured in metres per second.
Formula
The formula for average speed is one of the simplest in kinematics. It is defined as:
Average speed = Total distance travelled ÷ Total time taken
This relationship works for any type of motion, whether the student walked at a constant pace or varied his speed at different times, the formula always holds true. It gives us one single value that summarises the overall journey.
This formula is relevant because the question does not ask about acceleration, instantaneous velocity, or the direction of motion. It is strictly about speed, which is a scalar quantity. That means we do not need to consider direction, as we would if we were calculating average velocity. We are only interested in “how fast” and not in “which way.”
Solution (solving the problem)
Let us now substitute the known values into the formula. Using the SI units, we have:
Average speed = Distance ÷ Time
Average speed = 3000 m ÷ 1200 s
Carrying out the division, we get:
Average speed = 2.5 m/s
That is the average speed in SI units. But sometimes it is helpful to convert this back to kilometres per hour, especially since the question originally gave distance in kilometres and time in minutes. To convert from m/s to km/h, we multiply by 3.6 (because 1 m/s = 3.6 km/h). So:
2.5 m/s × 3.6 = 9 km/h.
Therefore, the student’s average speed is 2.5 m/s or 9 km/h.
Final Answer
The student’s average speed is 2.5 metres per second (m/s) when expressed in standard SI units. This is the most scientific way to state the answer, as it follows international conventions used in physics.
Alternatively, the average speed can be expressed as 9 kilometres per hour (km/h), which is often easier to interpret in day-to-day situations. Both answers are correct, and they are simply two ways of writing the same physical reality.
Helpful Explanation
To understand what this answer really means, imagine the student walking on a straight path. If instead of walking with a changing pace, he had walked steadily at exactly 2.5 m/s, he would have covered the 3 km in exactly the same 20 minutes. Average speed essentially “flattens out” all variations and gives us a uniform motion picture. That’s why it is such a useful tool in physics, because it simplifies real-world motion into a single, easy-to-handle value.
A common pitfall in problems like this is forgetting to convert units. If a student had divided 3 km by 20 minutes directly, they might have ended up with an answer like 0.15 km/min, which is awkward and less meaningful. Another mistake is mixing hours and minutes, which can lead to errors in the factor of 60. The tip is always: convert everything into SI units first, solve neatly, and then convert to other units if required. That way, you avoid confusion and keep your answers reliable.