Final answer:
Whether a needle floats or sinks in water is primarily influenced by surface tension. A clean cold needle can still float on warm water, as temperature differences do not significantly alter the ability to float if the needle is placed carefully.
Step-by-step explanation:
The behavior of a needle on the surface of water is governed by the concept of surface tension, which is a physical property of liquids that allows them to resist an external force due to the cohesive nature of their molecules. Figure 2.15 and other provided figures describe how the weight of a needle pulls the surface tension downward, but at the same time, surface tension pulls it up, suspending the needle on the surface of the water and keeping it from sinking. It is also critical that the needle is very clean, as contaminants like oil can affect the surface properties of the needle, making it more likely to sink.
Whether a needle is hot or cold does not directly affect its ability to float. However, the temperature of the water can influence the surface tension, with warmer water typically having slightly lower surface tension than colder water. Despite this, a clean, cold needle can still float on warm water if placed carefully, as the difference in surface tension due to temperature is usually not enough to cause the needle to sink, assuming that other conditions for floating are met (such as cleanliness and careful placement).