Why do objects float in Fluids?

Why do objects float in Fluids?

Eureka! The reason why objects stay afloat…for the most part

Continuing on our road down physics concepts, there is another fluids mechanics concept that is of importance, which is known as buoyancy. Ever wondered why a ball floats on top of water? Or why a boat doesn’t sink to the bottom of the ocean? This is all due to buoyancy. Buoyancy is by definition the force exerted onto an object that is equal to the weight of a displaced fluid and acts in the opposite direction as gravity.

Buoyancy and floating objects

Why do some objects float and others sink?

Good question! Well it mainly has to do with the differences in an object’s density. The buoyancy equation is:


Where ρ is the density of the fluid, V is the volume of water displaced, and g is gravity. Looking at this, we can see that changing the density of the fluid will vastly affect the buoyancy force. If the object’s density is greater than the density of the fluid, then that means the weight of the object is greater than the buoyancy force therefore the object will sink. If the object’s density is less than the fluid, the buoyancy force ends up being greater than the weight of the object, therefore allowing the object to float.

An Oddity in Floating Objects

You might be wondering then, why is it that sometimes some objects, like paperclips, can float on top of the water, but if they are dropped in water, they don’t float to the surface. That actually comes from surface tension. The idea behind surface tension is that the water can act like a thin sheet of paper that is very weak. The paper holds together, but if something with enough weight comes along, it will tear. The surface of water acts just like that.

Truly floating objects are actually in the water, instead of just on top of it. This is the biggest difference between floating in water and resting on water.