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Question

A ball is thrown up from the top of a tower at 20 m/s velocity. It took the ball 6 sec to fall to the bottom.  For how long was the ball above the tower?


  1. 2 sec

  2. 4 sec

  3. 6 sec

  4. 1 sec


Solution

The correct option is B

4 sec


When the ball was going up till B then its displacement was increasing.

When the ball started to fall from B to C its displacement was still positive but decreasing when ball is at C its displacement from the starting position is 0.

So when the ball comes back to C after falling from B then its displacement becomes O

So, we know ball was thrown with 20 m/s up

                                u = + 20 m/s

                                s = 0

Acceleration         = - 10 m/s2

As always the bell is under the influence of gravity

So a=10m/s2

s=ut+12at2

o=20.t+12×(10)t2

t(20 - 5t) = 0

t = 0 or 4 sec

The equation gives 2 answers for t at which s = 0

By analyzing we see that at t = 0 particle was at starting point only so displacement = 0

At, t = 4 s = 0

Means ball comes back to starting point after its journey.

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