The correct option is **B. The surface tension of the water creates a force that counteracts the weight of the needle.** ### Detailed Explanation: 1. **Understanding Density and Buoyancy**: - Density is defined as mass per unit volume. A metal needle is denser than water, meaning it has more mass in the same volume compared to water. - Buoyancy is the upward force exerted by a fluid that opposes the weight of an object immersed in it. According to Archimedes' principle, an object will float if the buoyant force is equal to or greater than its weight. 2. **Surface Tension**: - Surface tension is a property of liquids that causes the surface to behave like a stretched elastic membrane. It arises from the cohesive forces between liquid molecules. - In the case of water, the molecules at the surface are attracted more strongly to each other than to the air above, creating a "skin" on the surface. 3. **Why the Needle Floats**: - When a needle is placed on the surface of water, it does not sink immediately because the surface tension of the water creates a force that can support the weight of the needle. - The needle's weight is counteracted by the upward force from the surface tension, allowing it to float despite being denser than water. 4. **Why Other Options Are Incorrect**: - **A. The needle creates a vacuum underneath it, reducing the buoyant force.** - This statement is incorrect because a vacuum would not be created under the needle. Instead, the needle rests on the water's surface, and the buoyant force is not reduced by any vacuum. Buoyancy is determined by the weight of the fluid displaced, not by the presence of a vacuum. - **C. The needle displaces a volume of water equal to its weight, allowing it to float.** - This option is misleading. While it is true that an object floats when it displaces a volume of water equal to its weight, the needle does not displace enough water to equal its weight due to its density. Instead, it is the surface tension that allows it to remain on the surface without sinking. - **D. The temperature of the water increases the buoyancy of the needle.** - This statement is incorrect because while temperature can affect the density of water (warmer water is less dense), it does not directly relate to the ability of the needle to float. The primary reason the needle floats is due to surface tension, not temperature. ### Summary of Key Points: - A metal needle can float on water due to surface tension, which counteracts its weight. - Density alone does not determine whether an object will sink or float; buoyant forces and surface tension play crucial roles. - Surface tension arises from cohesive forces between water molecules, creating a supportive "skin" on the water's surface. - Understanding the principles of buoyancy and surface tension is essential for explaining floating behavior in liquids. This thorough understanding of why the needle floats will help you grasp the concepts of buoyancy and surface tension in physics.