First Question: Force applied for a span of time.
Let's say a force $F$ is applied on an object of mass $M$. So, the acceleration of that object in the direction of $F$, would be, $a = F/M$.
Now, if we apply the force for 5 seconds, what will be the acceleration on the object? Will we take the net force exerted on the object to be $5F$ and thus the acceleration would be $5a$?
Second Question: Objects falling after thrown
Let's say I have a ball on my hands. Right now, the only acceleration being applied on the force is the gravitational acceleration $g$, which is downwards (towards to ground). Now, if I apply force $F$ on the ball directly upwards, creating acceleration $a$ in the ball, the net acceleration upwards will be $a-g$.
If I were to be in a vacuum without any air resistance, the ball would keep moving upwards with acceleration $a-g$. But due to the air resistance (the ball hitting the air particles while trying to go upwards), the upwards acceleration $a$ decreases over time and at some point $a=g$ and $g>a$ afterwards. Then, the net acceleration would be downwards thus the ball will start falling to my hands.
Is this the right thought process or am I getting something majorly wrong?