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A stone falling under Gravity. The total distance covered in the last second of its journey equal to the distance covered by it in the first three seconds of motion. The time for which the stone is in air is
a) 5sec
b) 12sec
c) 15sec
d) 8sec
Answers & Comments
S(n) = u + 1/2*a*(2n - 1)
where, u = initial velocity, a = acceleration of the body
Let the stone remain in the air for t sec.
As, the stone falls from rest,
So, u = 0 m/s
Also, a = -g (acceleration due to gravity is directed downward) = -10 m/s^2
So, distance covered by the stone in the last second (or t th second) is :
S(t) = u + 1/2*a*(2t - 1)
= 0 - 1/2*10*(2t - 1)
= -5(2t - 1)
Distance covered by a body in n seconds is :
S = u*n + 1/2*a*n^2
So, distance covered by the stone in first 3 seconds is:
S = u*3 + 1/2*a*3^2
= 0*3 - 1/2*10*9
= 0 - 45
= - 45 m
Given,
S(t) = S
or, -5(2t - 1) = -45
or, 2t - 1 = 9
or, t = 5 sec
So, the stone remained in the air for 5 seconds.
Hope, it helps.
Distance covered by a body in the n th second is :
S(n) = u + 1/2*a*(2n - 1)
where, u = initial velocity, a = acceleration of the body
Let the stone remain in the air for t sec.
As, the stone falls from rest,
So, u = 0 m/s
Also, a = -g (acceleration due to gravity is directed downward) = -10 m/s^2
So, distance covered by the stone in the last second (or t th second) is :
S(t) = u + 1/2*a*(2t - 1)
= 0 - 1/2*10*(2t - 1)
= -5(2t - 1)
Distance covered by a body in n seconds is :
S = u*n + 1/2*a*n^2
So, distance covered by the stone in first 3 seconds is:
S = u*3 + 1/2*a*3^2
= 0*3 - 1/2*10*9
= 0 - 45
= - 45 m
Given,
S(t) = S
or, -5(2t - 1) = -45
or, 2t - 1 = 9
or, t = 5 sec
So, the stone remained in the air for 5 seconds.
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