I have a first order differential equation question with initial value. d

Celia Lucas

Celia Lucas

Answered question

2022-06-21

I have a first order differential equation question with initial value.
d v d t + v 2 5 = 9.8  with  v ( 0 ) = 0
I tried to solve it but just didn't know how to remove the absolute sign in my solution.
Please let me know if whole question is needed to remove the absolute sign inside the solution.
Edit 1: Part where I got stuck:
After integration and since C = 0:
5 l n ( l v + 7 l ) 14 5 l n ( l v 7 l ) 14 = t
Edit 2: I think my solution is wrong

Answer & Explanation

lisicw2

lisicw2

Beginner2022-06-22Added 11 answers

I'll write 9.8 as g for convenience.
d v d t = g 1 5 v 2 = 1 5 ( 5 g v 2 )
d v 5 g v 2 = d t 5
d v 5 g v 2 = d t 5 = t 5 + c
We know that 1 a 2 x 2 d x = 1 a tanh 1 ( x a ) so
1 5 g tanh 1 v 5 g = t 5 + c
tanh 1 v 5 g = g 5 t + c
v = 5 g tanh ( g 5 t + c )

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