Based on the information in the question, a light year is the distance traveled light in one year. Then if the light from the star Centauri takes 4.2 years to reach the earth, then its distance from earth is 4.2 light years.
In the, golf the ball was hit by a club with certain force. As the club hits the ball it's the action. When the ball flies away its the reaction.
When a person swings a golf club at the ball, when it hits the ball, it causes the ball to roll up the face of the club and into the air towards the target.