Question

2. A ball is dropped from the top of a 20 m building at the same time that another ball is thrown upward from a height of 2 m above the ground. If the two balls pass on an other at the height of 10 m, determine the speed at which the second ball was thrown upward Answer | 12.6 m/s.

Answer 1

Concept used: - equations of kinematics were used for solution.

Let at time t, both pass each other, then, for the first ball, displacement S = 20m - 10m = 10 m S = nu_L t + 1/2 gt^2, nu_L = 0 (taking down as positive) or, t = squareroot 2S/g = 1.43 s. upward displacement for 2nd ball is, h = 10m - 2m = 8m So, h = nu_L t - 1/2gt^2 (taking up as positive) or, 8m = nu_L t - 1/2(9.8 m/s^2)t^2, put t = 1.43 s or, nu_L = 12.6 m/s - speed at which 2nd ball was thrown up.
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