A bus starting from a bus-stand and moving with uniform acceleration attains a speed of 20 km/h in 10 minutes. What is its acceleration ?
- A.200 km/h²
- B.120 km/h²
- C.100 km/h²
- D.240 km/h²
Correct answer
Explanation
The correct answer is B, 120 km/h². Acceleration is the change in velocity divided by the time taken, and 10 minutes is one-sixth of an hour.
The bus starts from rest, so its initial velocity u = 0 and its final velocity v = 20 km/h. The time must be in hours to match the unit of speed: t = 10 minutes = 10/60 h = 1/6 h. Using a = (v − u)/t, a = (20 − 0) ÷ (1/6) = 20 × 6 = 120 km/h². The trick in such questions is to keep every quantity in the same units before dividing.
Option A, 200 km/h², is wrong: it comes from dividing 20 by 0.1, treating 10 minutes as a tenth of an hour. Option C, 100 km/h², is wrong because it matches no correct conversion. Option D, 240 km/h², is wrong: it would need 20 km/h in 5 minutes.
Exam tip: a = (v − u)/t; convert minutes to hours (divide by 60) before using km/h.
Practice Questions
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