Final answer:
Surface tension is important in the floating of a plastic sphere in water and the floating of a needle on water's surface.
Step-by-step explanation:
Surface tension is an important phenomenon in which the surface of a liquid acts like a stretched elastic sheet. It is significant in the floating of a plastic sphere in water, as the cohesive forces between the water molecules keep the sphere on the water's surface. Surface tension also plays a role in the floating of a needle on water's surface, as the forces in the stretched surface support the needle's weight. However, it is not important in the floating of a boat in water or the floating of a balloon in air.