Answer
a. Two force pairs act on the baseball while it is being hit: Earth’s downward pull on the baseball (action 1), and the bat's force on the baseball (action 2). During that time interval, it is momentarily at rest and air resistance is negligible.
The corresponding reaction forces are: baseball’s upward pull on Earth (reaction 1), and the baseball's force on the bat (reaction 2).
b. In flight, two forces acts on the baseball: Earth’s downward pull on the baseball (action 1), and air drag pushes on baseball (action 2).
The corresponding reaction forces are: the baseball’s upward pull on the Earth (reaction 1), and the baseball pushing the air (reaction 2).
