Concept Check – Data needed to find speed when crossing a stationary engine: A train crosses a stationary engine in 20 seconds. To compute the speed of the train, which information is necessary?

Introduction / Context:
When an object (train) passes another object with non-negligible length (engine), the total distance covered relative to the engine during the pass is the sum of their lengths. With the time known, we need the total distance to get speed.

Given Data / Assumptions:

• Passing time t = 20 s.
• Engine is stationary but has length.

Concept / Approach:
Speed v = (L_train + L_engine) / t. Therefore, to compute v numerically, we must know L_train + L_engine, which requires both lengths (unless one length is zero, which is not the case here).

Step-by-Step Explanation:
Identify that distance covered during pass = combined length.With only one length, the other is unknown; hence total distance is undetermined.Therefore both lengths are necessary.

Why Other Options Are Wrong:
Options stating “only” one length or “either” length ignore that the sum is required; “neither” is incorrect because some length data are needed to compute distance.

Common Pitfalls:
Confusing this with passing a pole (where only train length matters) and assuming the same rule applies when the other object has length.

Final Answer:
Both the length of the train and the length of the engine