Answer:
First, we need to remember the second Newton's law:
This says that:
F = m*a
Force equals mass times acceleration.
This means that if we apply a force to an object, the object will accelerate.
This also means that if an object is not moving, or moving with a constant velocity, it will keep doing that until a force is applied.
Now let's answer the 3 cases:
a. The baseball is in space and stationary. What will the ball do? Nothing, it will keep being stationary Why? There is no force applied, or the net force is equal to zero.
b. The ball is in space and someone hits it with a bat. What will the ball do? Will move with constant speed Why? A hit with a bat can be thought as a great force applied for a really short period of time, this means that the speed of the ball will increase a lot, but because the force only last for a really short time, after that the ball will keep moving with constant speed.
c. The ball has a small jet pack attached that is continuously firing. What will the ball do? The ball's speed will keep increasing more and more. Why? Now we have a constant force, this means that we will have a constant acceleration, and as we know, the acceleration is the rate of change of the speed, this will mean that the speed of the ball will keep increasing until the jet pack stops firing, and after that, the ball will keep moving with constant speed.
