Question

an athlete completes one round of a circular track of radius r in 40 seconds.what will be his displacement at the end of 2 minutes 20 seconds

Answer 1

Total time duration in consideration : 2 min 20 secs = 140 secs

No of complete rounds in this time = 140/40 = 3 rounds

After these 3 rounds, he will be exactly at the point where he started. Hence his displacement will be 0, although he would have covered a distance of 3 (2πR)

Now in the next 20 secs, he will have covered half the circular track (assuming he travels at a uniform speed), So his dipslacement will be $2 \times r$ (he will be diametrically opposite to where he started from and displacement is the shortest distance between initial and final positions)

Answer 2

after 140 second, he will be on the diametrically opposite point of starting point. so displacement is the diameter or 2R.