Question

a ball is thrown upwards with a velocity of 100m/s. it will reach the ground......

10s
20s
5s
40s

Answer 1

when ball is thrown upward =>
initial velocity (u)=0
final velocity (v)=100 ms-1
distance =?
By kinematical equation ;
v^2 =u^2+2as
v^2 =2as . . . . . u=o
s = v^2÷2a
s =10000÷20
s=500 m
Finding time to reach at top ::
v =u+at
v=at
t=v/a
t=100/10
t=10 seconds

Now,
when ball will be at top ;
u=0
v=acceleration due to gravity (g)=10 ms-2

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therefore,
By second kinematical equation,
s =ut + 1/2gt^2
u=0
t^2=2s/g
t^2=2×500/10
t^2=100
t= 10 second
Time taken to come down = 10 seconds
ANS.
Total time =10+10= 20 seconds .

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Hope it will help