Now that you have followed the first two articles in this series, you can create momentum (Newton’s First Law) by pushing back against the pool wall to move forward (Newton’s Third Law) with a streamlined body (First).

However, your forward momentum won’t last long unless you apply Newton’s Second Law. That would be the Law of Acceleration: Acceleration of a body by an outside force is directly proportional to the amount of force exerted, the direction in which the force is applied, and the length of time that the force is applied.

Increased propulsion means applying a force to add to the swimmer’s momentum. That primary force is an arm stroke that levers the body past the hand (and arm) anchored against non-moving water.

It thus reasons that most professional swimmers are muscular with long arms and big hands that can deliver a strong force to propel the body past the anchored hand.

Newton’s Third Law (action-reaction) would seem to indicate that the stroke be directed straight backward to propel the body forward, but in water this is not possible. Because there is no traction against water once it is moving, the leverage can’t be in a perfectly straight line.

The lever has to move subtly up-down-in-out in a slightly curvilinear fashion to make continuous contact with non-moving water. The surfaces of the hand, lower arm, and upper arm will still mostly be facing backward as the swimmer maneuvers for traction.

To apply force for the longest time, the swimmer needs to catch the water with an extended high elbow (EVF or Early Vertical Forearm in “coachspeak”) and continue the stroke until the thigh reaches the anchored hand.

This is true for all strokes except breaststroke, where the recovery underwater would create too much resistance. But breaststroke compensates with its kick; the bottoms of the feet and insides of the legs also put pressure on the water for as long as possible.

Efficient swimming is primarily a product of applying all three of Newton’s Laws of Motion to achieve relaxed but effective strokes that propel the swimmer through the water.

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.