@MatinSAR, in addition to what has already been said, here's an attempt to explain the mistakes (and omissions) in your original answer.
I'm assuming only a qualitative answer was required, not an equation for F as a function of time.
MatinSAR said:
My answer : According to the question, the glass and the air inside it entered the water. Let's assume that the net force becomes zero at a moment,
You have not clearly explained that, because the glass moves at constant velocity, its acceleration is zero. And since ##F_{net}=ma##, this means the net force must always be zero while the glass moves downwards through the water.
MatinSAR said:
that is, the sum of the weight force and F is equal to the buoyancy force.
The weight force (##\vec W##) and ##\vec F## both act downwards. Buoyancy (##\vec B##) acts upwards. So ##\vec W+\vec F = \vec B## can never be true. You mean the
magnitudes of these forces give ##W + F = B##.
MatinSAR said:
By going down in the water, the gas volume decreases,
Correct. Though you haven't explained why the gas volume decreases.
MatinSAR said:
so the buoyancy force also decreases
Correct. Though you haven't explained why the buoyancy force decreases.
MatinSAR said:
, as a result, F also decreases.
OK.
Edit. Do you think it's possible that, at some depth, F would be zero? If so, consider what happens after that point and how F would have to change in order to keep the velocity constant!
MatinSAR said:
From time to time, the gas volume stops decreasing, as a result, the buoyancy force and F remain constant.
That’s just plain wrong, as others have noted. This is a smooth, continuous process. The gas volume will smoothly and continuously decrease . Why would you think otherwise?
So your conclusion about how F and B change is also wrong.
MatinSAR said:
So F decreases after equilibrium
What do you mean 'after equilibrium'? The descending glass is
always in equilibrium (zero acceleration, so zero net force). I think you mean 'So F decreases after the glass enters the water.'.
MatinSAR said:
and remains constant after stopping the decrease in volume.
No. F does not remain constant and the volume never stops decreasing. Your conclusion based on incorrect assumptions.
MatinSAR said:
Can someone tell me Why my answer is wrong ?!
See above! Also, you haven't really answered the question about F. Without equations, one way to describe what F does would be to sketch a graph of F's magnitude vs. time. Can you do this?
Edit. minor wording change