Final answer:
Replacing a ping-pong ball with a lighter object of the same size and shape will result in a reduction of tension in the string during rotation because the tension comprises both weight and centripetal force components and both will be smaller for a lighter object.
Step-by-step explanation:
When the ping-pong ball is replaced by an object of the same size and shape but with a lesser weight, the tension in the string during rotation will also be less at any point in the vertical circle. This is because the tension in the string at any point is the result of the centripetal force required to keep the object moving in a circle and the weight of the object.
If the ball is moving at the slowest possible speed at the top of the circle, the tension there is essentially zero because the only force acting on the ball is gravity, which provides just enough force to keep the ball moving along the circular path (centripetal force). However, at the bottom of the circle, the tension is at its maximum because it must balance both the weight of the object and provide the centripetal force necessary to keep the object in circular motion. As such, if a lighter object is used, not only is the weight component reduced, but the required centripetal force is also smaller, leading to a decrease in tension compared to that for a heavier object.