Final answer:
The submerged ping-pong ball in water demonstrates buoyancy, as the ball tends to rise due to the upward force exerted by the water, showcasing that it is less dense than the fluid.
Step-by-step explanation:
The ping-pong ball being held submerged in a bucket of water by a string demonstrates the concept of buoyancy. Buoyancy is the upward force exerted by a fluid that opposes the weight of an object immersed in it, allowing objects less dense than the fluid, like a ping-pong ball, to float. When the ball is held under water, it shows that even though it is being pulled down, it has a tendency to rise to the surface due to this buoyant force, a behavior opposite to that of a needle resting gently on the surface due to surface tension.
Surface tension is another phenomenon that occurs at the surface of a liquid, where the liquid acts as if it's covered by an elastic membrane. However, in the case of the ping-pong ball, surface tension is not the main force at work since the ball would ordinarily float on its own due to its lower density compared to water. The submerged ping-pong ball demonstrates buoyancy rather than surface tension, which would be demonstrated by the floating needle or water strider that can stay afloat because of the cohesive forces at the liquid's surface.