WAEC 2010 Physics · Question 2
Two cars X and Y travelling in opposite directions along the same high way at uniform velocities 110 kmh^ -1…
Question
Two cars X and Y travelling in opposite directions along the same high way at uniform velocities 110 and 90 respectively pass each other at a certain point. The velocity of X relative to Y at the time they pass each other is
- A.200
- B.100
- C.40
- D.20
Answer: A. 200
Explanation written with AI and checked against the answer key
Analyze the question: Two cars move in opposite directions along the same road. We need the velocity of X relative to Y, which is how fast X appears to move to someone sitting in Y.
Concept: Relative velocity of X with respect to Y is
v(XY) = v(X) − v(Y)
Velocity is a vector, so direction matters. When two objects move in opposite directions, their speeds add.
Solution:
Take the direction of X as positive.
- v(X) = +110 km/h
- v(Y) = −90 km/h (opposite direction)
v(XY) = 110 − (−90) = 110 + 90 = 200 km/h
Check: if you were in car Y, car X would seem to rush toward you and past you at the combined speed of both cars, which is 200 km/h.
Answer: A. 200 km/h
Tip: For relative velocity questions, check the directions first.
- Opposite directions: add the speeds.
- Same direction: subtract the speeds.
Options C (40) and D (20) are the traps for students who subtract when the cars are moving in opposite directions.
AI can make mistakes. Report anything that looks wrong.
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