Final answer:
Gravity is the force that causes a tennis ball to follow a curved path when hit into the air, constantly acting on the ball throughout its flight and resulting in its arc-shaped trajectory.
Step-by-step explanation:
The force that causes a tennis ball to follow a curved path when hit upward into the air is gravity. When the ball is moving upwards, it is slowed down by gravity until its vertical speed becomes zero, and then gravity causes it to accelerate back down to the ground. This constant force acts on the ball throughout its flight, resulting in the characteristic arc of its trajectory.
Comparatively, Newton's law of universal gravitation explains the paths of planets moving around the sun, among other phenomena, due to the gravitational force acting between masses. Other forces such as friction, elastic force, tension, and electric forces also play important roles in different contexts within physics. For instance, electric forces are responsible for holding an atom together.