1) An object at rest tends to remain at rest, and an object in motion tends to continue moving with a constant velocity, until acted upon by a net external force.
* The tennis ball is moving in your direction, and it continues moving in your direction until you apply a force by striking it with the racquet.
2) Acceleration equals force over mass (or "force equals mass times acceleration")
* The harder you hit the ball, the more quickly it moves away from you, because you impart a greater acceleration to it.
3) An action force of object A upon object B is accompanied with a reaction force equal in magnitude and opposite in direction, of object B upon object A.
* The racquet exerts a forward force upon the ball, and thus the ball exerts an equal backward force upon the racquet, causing it to recoil from the impact.