Question

NCERT science chapter 8 page 112 class 9th

Answer 1

Given, Diameter (d) of the track = 200m

Time taken for 1 round of the track = 40s (t)

Distance travelled in 1 round (s) = circumference of track

= 2π r

= πd

= π x 200

(s) = 200π m

Hence, speed of athlete along the track (V) = s/t = 200π/40

= 5π m/sec

Time interval = 2 minutes 20 sec (Given)

= 140 seconds

∴ distance travelled in 140 sec is given by s = v x t

= 5π x 140

= 5 x 22/7 x 140

= 2200 m

The no of revolutions completed = 2200m/200π.

= 2200/(200x22/7) = 3.5

Athlete has completed 3.5 revolutions.

∴ Displacement of athlete = diameter of track = 200m.