Density and Buoyancy. Density and Buoyancy. Buoyancy is the tendency to rise or float in water or air. The upward force exerted on objects submerged in fluids is called buoyant force .
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According to the particle theory, different substances have different sized-particles. The size of the particles determines how many particles can fit into a given space. Therefore, each substance has a unique density, based on particle size.
An airship such as a blimp can float because its mass is relatively small compared to its enormous volume. Therefore, its average density is slightly less than the density of the air surrounding it. Ocean-going ships, hot-air balloons, and blimps all have huge volumes.
The relationship between the size of an object and the buoyant force exerted upon it was established long ago by a scholar named Archimedes.
The particle theory can explain why this was a reasonable conclusion.
If the water in a container is still, or at rest, then the water particles are neither rising nor sinking. An object immersed in a fluid such as water doesn’t rise or sink if the amount of force pulling down (gravity) equals the amount of force pushing up (buoyancy).
If 1L of water is displaced, the object replacing it would have the same volume (1L), but might have a different weight than the 1L of water. If the object is heavier than the water, then the weight of the object will be a force greater than the buoyant force that had supported the displaced water. Therefore, the object will sink.
If the object is lighter than the displaced water, the object will rise to the surface and float.
The buoyant force of a liquid does not depend on physical state, but rather on density.
Objects float more easily in salt water than in fresh water. Salt water has a density of 1.03g/mL and fresh water has a density of 1.00g/mL.
The density of salt water is greater than that of fresh water, which means that the particles are packed together more tightly. Therefore, salt water can support more weight per volume than fresh water.