Reader Questions

Reader Question: Swimming and Buoyancy

Differences in center of mass and center of buoyancy can affect stability

aniiika asks:

How does buoyancy relate to swimming?

Buoyancy is the force that enables a swimmer to float in the water, even when still. Buoyant force is equal to the weight of the fluid displaced by the swimmer; in other words, the density of the fluid multiplied by the volume of the swimmer that is submerged.

Different people float at different heights in the water depending on many factors, such as body shape, amount of fat, and how much air is in their lungs. All of these things affect a person’s volume and/or density, thereby affecting the buoyant force they experience.

Because a person’s body is not fully submerged their center of buoyancy–the point where all buoyant forces on the body can be represented by a single force–does not coincide with the center of mass (sometimes referred to as center of gravity). Where those forces are relative to one another determines the stability of a person floating in the water. Everyone’s center of buoyancy is higher than their center of mass, so people always float stably in an upright orientation. Our legs, for example, don’t float as well as our torsos, so, when floating horizontally, one’s legs will tend to sink.

Swimmers can control their buoyancy to their advantage by actually pressing their upper chests further into the water. This tends to bring one’s hips closer to the surface and can reduce drag (#).

One comment
  1. TD

    Dear Answer Person,

    “Because a person’s body is not fully submerged their center of buoyancy–the point where all buoyant forces on the body can be represented by a single force–does not coincide with the center of mass …”
    I believe that even if a person were fully submerged their center of buoyancy would not coincide with the center of mass. Our center of mass is not at the geometric center of our body — but for a fully-submerged person, the center of buoyancy would be at the geometric center of the body.

    I think your illustration is very helpful

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