A needle sinks because it's denser than water, while a ship floats because, despite being made of steel (which is denser than water), its shape allows it to displace a large volume of water, effectively making its overall density less than water.
Here's a breakdown of the principles at play:
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Density: Density is mass per unit volume. If an object's density is greater than water, it sinks. If it's less dense, it floats.
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Archimedes' Principle: This principle states that the buoyant force on an object submerged in a fluid is equal to the weight of the fluid displaced by the object.
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Buoyancy: Buoyancy is the upward force exerted by a fluid that opposes the weight of an immersed object. If the buoyant force is greater than the object's weight, the object floats.
Let's consider each object:
The Needle
- A needle is made of steel, which has a significantly higher density than water.
- When placed in water, the needle displaces a small volume of water. The weight of this displaced water is less than the weight of the needle itself.
- Therefore, the buoyant force is less than the needle's weight, and the needle sinks.
The Ship
- A ship is also made of steel (or other dense materials). However, it's designed with a large, hollow hull.
- This design allows the ship to displace a much larger volume of water than a solid block of steel with the same weight.
- The weight of the water displaced by the ship's hull is greater than the weight of the ship itself (including its cargo and passengers).
- Therefore, the buoyant force is greater than the ship's weight, and the ship floats. The ship floats as long as the weight of the ship and its contents is less than the weight of the water it displaces.
In essence, the ship's shape increases its volume without significantly increasing its mass. This reduces its overall density (including the air inside) to less than that of water.
