I am have trouble with this problem. MY study mates and I got3 different answers

Question

I am have trouble with this problem. MY study mates and I got3 different answers. could you please help us. A tennis ball is shot vertically upward from the surface of anatmosphere- free planet with an initial speed of 20.0m/s. onesecond later, the ball has an instantaneous velocity in the upwarddirection of 15.0m/s. How long does it take the ball to reach its maximumheight? a-2.3s b-4.0s c-8.0s d-2.0s e- 4.6s I think the answer is a. I am have trouble with this problem. MY study mates and I got3 different answers. could you please help us. A tennis ball is shot vertically upward from the surface of anatmosphere- free planet with an initial speed of 20.0m/s. onesecond later, the ball has an instantaneous velocity in the upwarddirection of 15.0m/s. How long does it take the ball to reach its maximumheight? a-2.3s b-4.0s c-8.0s d-2.0s e- 4.6s I think the answer is a.

Explanation / Answer

given intial speed of ball = u = 20.0m/s and after one seconed velocity of ball = v = 20m/s    then from the kinematic eq = v = u+at then a = v-u/t            =15-20/1 = -5m/s2 if the ball is moving this constant deaccleration then v = u+at    final velocity at maximum heigth = v = 0 then 0 = 20m/s +(-5m/s2) t so t = 20m/s/5m/s2 = 4.0s so b is correct answer . thank you i hope this is help you.

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