Ans:
According to the question, the initial velocity of the ball (u) is 49 m/s
Case 1 : When the lift is stationary, the boy throws the ball upwards. The positive direction is considered to be vertically upward. The ball’s displacement is zero.
Making use of the equation of motion => s = ut + (1/2)at2
Substituting s = 0, u = 49 m/s and taking a = -g = – 9.8 m/s2 , we get
0 = (49)t + (1/2) (-9.8)t2
Upon re-arranging => t = (49 x 2)/9.8 = 98/9.8
Therefore, t = 10 sec
Case 2 ; In this case, since the lift starts moving with a velocity of 5 m/s, the initial velocity of the ball becomes
Thus, urelative is equal to the velocity of the ballTherefore the time taken in this case is given by the following expression ;t =2urelativegrelative=2×499.8t=10 seconds
This shows that the time taken is same in both the cases.