While this column introduces another bit of physics applicable to swimming, let me say first that a swimmer doesn’t need to know physics. It’s just helpful to know that a lot of an instructor’s simple “Do this” or “Do that” directions might have a scientific basis.

Introducing Bernoulli’s Principle: As the speed of a fluid increases, the pressure in the fluid decreases.

This explains, for example, how airplanes stay aloft, because the pressure under the wing is greater than in the air that flows over it.

You can feel this same lift with your hand out a car’s window (but not the driver’s window). And that lift is possible in the water, too, when you consider a swimmer’s hand (and lower arm) to function as a wing. Lift fits all strokes, but is more easily demonstrated with the breaststroke.

Start vertical in the water, which optimizes forward resistance. Extend the arms forward with the fingertips angled down toward the pool bottom. Move the hands and lower arms out and in, feeling greater pressure on the palms than over the backs of the hands.

There is thus no part of the body that is directed backwards to move the body forward according to Newton’s Third Law (drag force). And yet this position and sculling action produces forward motion.

Stroking propulsion, therefore, is achieved through a combination of drag force and lift force – in proper physics terms, a resultant vector. Drag and lift forces vary in proportion from stroke to stroke and also at various points during the stroking process of moving the body past the anchored hand and forearm.

Both forces require a steady application of increasing pressure.

Again, please don’t get the impression that good swimming requires knowledge of physicists (Newton, Froude, Bernoulli) and their principles. All these recent columns mean is that there are scientific reasons behind an instructor simply saying “Do this or that” for a swimmer to become more efficient.

Dr. Bob Colyer of Bluffton is an actively retired college professor, coach and author of “Swim Better: A Guide to Greater Efficiency for Swimmers & Instructors,” directed primarily to non-competitors. bobcolyer@yahoo.com