Difficulty: Easy
Correct Answer: 8 sec
Explanation:
Introduction / Context:
This is a basic train and pole question, serving as a foundation for more complex problems. When a train crosses a pole, it needs to cover only its own length. With the train length and its speed given, we can directly use the distance and speed relationship to find the time taken to cross the pole completely.
Given Data / Assumptions:
Concept / Approach:
For a train crossing a pole, distance = length of the train. We convert the speed from km/h to m/s, ensuring that units match. Then we apply the formula time = distance / speed to get the answer in seconds. This is a direct one step calculation once units are consistent.
Step-by-Step Solution:
Step 1: Convert 72 km/h to m/s.
Step 2: 72 km/h = 72 * 5/18 = 20 m/s.
Step 3: Distance to be covered to cross the pole = train length = 160 m.
Step 4: Use time = distance / speed.
Step 5: Time = 160 / 20 = 8 seconds.
Verification / Alternative check:
We can think in reverse. If the train takes 8 seconds at 20 m/s, the distance covered is 20 * 8 = 160 m, which exactly equals the given train length. This perfectly matches the definition of crossing a pole, confirming that 8 seconds is correct.
Why Other Options Are Wrong:
A time of 4 or 6 seconds would imply higher speeds than 72 km/h for the same train length, while 9 or 10 seconds would imply lower speeds. Since the speed is fixed at 72 km/h, these alternatives are inconsistent with the distance and speed relationship.
Common Pitfalls:
Learners sometimes forget to convert km/h to m/s or accidentally use 72 / 20 instead of 160 / 20 when computing time. Additionally, some confuse the distance for a pole with that for a platform or another train, where more than one length is involved. Being clear about what the train is crossing avoids such confusion.
Final Answer:
The train will take 8 sec to cross the electric pole completely.
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