Answer:
Here, S = 20 m. Using; \[{{\upsilon
}^{\text{2}}}={{\text{u}}^{\text{2}}}+\text{2as}\]. For
upward motion, we have
\[0={{\text{u}}^{\text{2}}}+\text{2}\times \left( -\text{1}0
\right)\times \text{2}0\]or \[\text{u}=\text{ 2}0\text{ m}/\text{s}\]. If t is
the total time of flight of the ball in going up and coming back, then total
displacement in time t is zero i.e, S = 0. Using the relation,
\[\text{S}=\text{ut}+\frac{1}{2}\text{a}{{\text{t}}^{\text{2}}}\],
we have
\[0\text{ }=\text{ 2}0\text{t }+\frac{1}{2}~\left( -\text{1}0
\right){{\text{t}}^{\text{2}}}=\text{ 2}0\text{t
}-\text{5}{{\text{t}}^{\text{2}}}\] or r = 4sec.
time interval of each ball = \[\frac{4}{4}\] = 1 sec.
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