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Motion In Straight Line

Question
CBSEENPH11026421
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A ball is dropped from the top of a building 100 m high. At the same instant another ball is thrown upwards with a velocity of 40 m/s from the bottom of the building. The two balls will meet after

  • 3 s

  • 2 s

  • 2.5 s

  • 5 s

Solution

C.

2.5 s

Let balls meet after t s. The distance travelled by the ball coming down is 

          s1 = 12 gt2

Distance travelled by the other ball

        s2 = 40 t - 12 gt2

         s1 + s2 = 100 m

∴        12 gt2 + 40 t - 12gt2 = 100m

                  t = 10040

∴                 t = 2.5 s

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