Final answer:
The ball bouncing off and returning to the ruler is due to the elastic force and air resistance, with the elastic force enabling the bounce-back motion and the air resistance affecting the ball's movement through the air.
Step-by-step explanation:
When a ball bounces off a barrier and returns to a ruler, the forces responsible for this are the elastic force and air resistance. The elastic force is the restoring force that allows the ball to bounce back; it is the same type of force that returns a plastic ruler to its equilibrium position after being bent and released, as described in the reference material. Air resistance, on the other hand, is a force that opposes the motion of the ball through the air, impacting its speed and trajectory as it moves and bounces back.