Q 11-02-011NEETJEE MainEAMCET 2010 (Engineering)Top questionEasy
An athlete completes one round of a circular track of radius $R$ in $40$ s. What will be his displacement at the end of $2$ min $20$ s?
Answer: (B) $2R$
Concept. Displacement is the straight line from start to finish; whole rounds bring the athlete back to the start.
Step 1. Number of rounds. $2$ min $20$ s $= 140$ s, so $\dfrac{140}{40} = 3.5$ rounds.
Step 2. Final position. After $3$ rounds he is at the start; the extra half round takes him to the diametrically opposite point.
Step 3. Displacement. The diameter, $2R$. (The distance $7\pi R$ is a trap.)
Answer. $2R$ (Option B).
Solution by Sreeraj P, M.Sc Physics