Terminal Velocity Given Acceleration at Instantaneous Velocity

[Greatness]

Homework Statement

This is a repost of a homework like question. The previous thread I did not understand.
In the assumption that drag is proportional to velocity, and when v = 20 m/s, a = 7.35 m/s^2, find the terminal velocity.

Homework Equations

The thread stated that the equation most relevant would be ma = mg - kv.

The Attempt at a Solution

I substituted all know values to make m(7.35) = m(9.8) - k(20), and attempted to solve, but the answer is 40 m/s.

 

[verty]

How far did you get? Did you get stuck anywhere? It is too soon to give a hint, not enough has been done.

 

[Greatness]

How far did you get? Did you get stuck anywhere? It is too soon to give a hint, not enough has been done.

None of that information is needed. The initial question is that it is dropped from a large height. The only information given is the information proveded (v and a).

 

[Greatness]

Never mind. I solved it by not replacing ma with 0 but by dividing by m first and replacing v with v^2.

In these types of problems, when is v or v^2 used?

 

[Greatness]

Hmm. In the initial problem, the mass of the steel ball mentioned in the initial problem is invalid. The mass is one if the answer is 40m/s with given info.
https://www.physicsforums.com/showthread.php?t=659382

Either way, the work is done.