When a stone is dropped from a building 200 m high, its speed is proportional to the time elapsed after dropping. The distance traveled is proportional to the square of the time elapsed. After 1 second the speed of the train was 10 m/sec and it was 190 m above the ground. When its speed is 25 m/sec, what would be its distance from the ground?

a) 140 m b) 137.5 m c) 125.75 m d) 142.5 m

• b) 137.5 m
  
  v= x*t and s= y*t^2
  10 = x*1, hence x=10 units
  (200-190) = y*1*1
  y=10
  
  when speed = 25m/sec, time elapsed = 25/10 = 2.5 sec
  
  distance travelled by stone = 10*(2.5)^2 = 62.5 mts
  
  so distance of stone above ground = 200-62.5 = 137.5 mtrs .. option b)
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