A note on the conservation of momentum in swimming

Today blog comes from the “notes” section of my new book (coming soon) Swimmings Book of The Five Rings, enjoy!

Throughout this book I reference “line” or “the line” and how important it is to understanding stroke mechanics, speed, and efficiency in the individual competitive strokes. Without getting into a textbook on hydrodynamics, let’s simplify the idea by looking at velocity and the principle of conservation of momentum.

In physics, conservation of momentum means that an object in motion will continue in motion unless acted upon by an external force. In the water, those forces are primarily drag and the resistance of the water. There are three main types of drag every swimmer battles:

• Form drag: caused by the shape and alignment of the body in the water.

• Frictional drag: caused by water moving along the skin and suit.

• Wave drag: caused by the disturbance we create on the surface.

Every technical error whether it’s a dropped hip, a misaligned head, or a late kick, creates more drag, and drag is what robs us of speed.

In a race, the peak of velocity occurs at one moment: when the swimmer leaves the blocks and flies through the air. From that instant forward, the race is a battle not to generate new speed but to hold on to the speed already created. Swimming is largely an art of managing inertia: once the body is moving fast, the key is to minimize the forces that slow it down.

“Easy speed” comes from technical efficiency, which is directly tied to how well we preserve momentum. A long, tight streamline off the walls reduces form drag. A consistent, narrow kick reduces wave drag. Even stroke mechanics, entry angle, catch position, and timing are about channeling force in line with forward velocity rather than leaking energy sideways.

This is why “the line” is so important. The line represents the body at its most hydrodynamic position, where resistance is minimized and velocity is best maintained. Every stroke and every movement should serve the return to that line.

When viewed this way, all biomechanics in swimming connect back to one truth: the race belongs not to the swimmer who thrashes the hardest, but to the one who best conserves momentum, the one who can best ride the line.

Study this.