Question
A driver is moving at a constant speed of 115 kilometres per hour when he receives a text message on his mobile phone. We are asked to determine how far he has travelled in 20 seconds after receiving the text. The question focuses on uniform motion, where the speed remains constant throughout the given time.
This type of problem is straightforward because there is no acceleration or deceleration to worry about. It is simply distance travelled = speed × time. The key step, however, lies in ensuring that the units are consistent. Since the speed is given in kilometres per hour while the time is given in seconds, we must carefully convert the units before performing the calculation.
Quick Answer
Distance = Speed × Time = (115 km/hr) × (20 s)
Convert speed into m/s: (115 × 1000) / 3600 = 31.94 m/s
Now multiply: 31.94 × 20 = 638.8 m
Convert back to km: 638.8 ÷ 1000 = 0.639 km
The driver is about 0.64 kilometres away after 20 seconds.
Understanding the question with detailed explanations
This problem is essentially about finding the distance travelled in a short time when the speed is known. The driver is already moving at 115 km/h when he receives the text, and we are assuming he continues at the same speed for the next 20 seconds. This is a common assumption in physics problems unless otherwise stated, because it simplifies the scenario into uniform motion.
The important thing to notice is the mismatch between units. Time is given in seconds, but the speed is in kilometres per hour. Without converting these properly, any direct multiplication would lead to meaningless numbers. Therefore, the whole question becomes an exercise in applying the formula correctly and handling units with care, which is an essential skill in physics.
A glimpse of the final answer
The calculation shows that in 20 seconds, the driver covers about 0.64 kilometres, which is equivalent to 639 metres. This is a surprisingly large distance for such a short period of time, but it makes sense because 115 km/h is a fairly high speed. In fact, it is faster than the speed limit on many urban roads, meaning that even a brief distraction can result in the driver travelling a significant distance without paying full attention.
By expressing the answer in both metres and kilometres, we gain two perspectives. In everyday terms, 0.64 km may sound small, but when you say 639 metres, it suddenly feels much longer — nearly two-thirds of a kilometre. This confirms the seriousness of how far someone can travel in just seconds at highway speeds.
Data
From the question:
• Speed of driver = 115 km/h
• Time = 20 seconds
What we need:
• Distance travelled after 20 seconds.
To work in SI units, we convert speed from km/h to m/s. Since 1 km = 1000 m and 1 hour = 3600 seconds, the conversion is:
Speed in m/s = (115 × 1000) / 3600.
Thus, speed = 31.94 m/s (rounded to 2 decimal places). The unknown we must calculate is distance, expressed either in metres or kilometres.
Formula
The general formula is:
Distance = Speed × Time
This formula applies because the driver is assumed to travel at a constant speed with no acceleration or deceleration during the 20 seconds. It is one of the fundamental equations of motion for uniform speed.
We are not concerned with velocity here, since direction does not matter; the problem is scalar in nature. The focus is purely on magnitude, how far the driver has gone in a given time.
Solution (solving the problem)
Step 1: Convert speed into metres per second.
115 km/h = (115 × 1000) / 3600 = 31.94 m/s
Step 2: Apply the formula.
Distance = Speed × Time = 31.94 × 20 = 638.8 m
Step 3: Convert back into kilometres for convenience.
638.8 m = 638.8 ÷ 1000 = 0.639 km
So, the driver travels approximately 0.639 kilometres in 20 seconds.
Final Answer
The driver covers a distance of 638.8 metres in 20 seconds. In kilometres, this is 0.64 km when rounded to two decimal places.
This means that in the short span of just 20 seconds, the driver has moved almost two-thirds of a kilometre, which is long enough to pass several street blocks in a city or cover a good stretch of a highway.
Helpful Explanation
This result highlights the importance of understanding speed and distance in real life. At highway speeds, the car moves nearly 32 metres every single second. That is longer than the length of most basketball courts in just one second. Over 20 seconds, the car goes more than six football fields in distance. If the driver’s eyes are off the road during that time, the risk of accidents increases dramatically.
A common mistake students make in such questions is failing to convert units properly. If someone multiplied 115 by 20 directly, they might think the answer is 2300 km, which is obviously unrealistic. The lesson is: always bring quantities into the same unit system before solving. In exams, double-checking units is as important as doing the arithmetic itself.