Forces
Swimming, like most sports, has evolved by leaps and bounds. Athletes today
can run faster, jump higher, and throw farther and faster then before. A better
understanding of the sports and ways to improve upon them has evolved as the
athletes have evolved. The same goes for swimming. Prior to the 1970’s,
it was believed that the best way to propel the body forward through the water
was to pull the hand directly backwards. This was the use of drag forces. This
drag force is opposite to the direction of the hand. Many believed that the
plane of the hand should be square to the direction of motion (perpendicular
the motion), and so it was taught.
As the sport evolved, the idea of square movement changed to curved paths. Good
swimmers now use sculling actions to utilize lift
forces. This is Bernoulli's Principle at work. The Principle of "foil-like"
objects moving through a fluid at high speeds with small angles to the flow
and a large lift forces is generated, while the drag forces are minimized. The
lift forces are caused by the fluid traveling further and faster around the
more curved side than the less curved side. Essentially, the hand acts as a
foil. This new method eventually became accepted and widely known.
(http://www.ifkb.nl/B4/indexsw.html)
Bernoulli's Principle is only one explanation of the kinetics of the lift force.
Drag and lift both contribute to the net force produced by the hand. Ideally,
the combination of lift and drag forces is such that the resultant force is
in the desired direction.
In the aquatic environment, propulsion is generated by accelerating water. The
momentum, P, of a mass of water, m, traveling with velocity, v, is P = mv. By
forcing water backward with a momentum, the resultant propels the swimmer forward.
(http://www.ifkb.nl/B4/indexsw.html)
The pushed-away mass of water acquires a kinetic energy ½mv2. This kinetic energy is the result of the work done by the swimmer on the pushed-away mass of water. Part of the total work of the swimmer is converted into kinetic energy of the water, rather than forward speed of the swimmer.
By combining these two ideas, a body is propelled through the water by giving
water a momentum in the opposite direction and propelling the body forward.
In order to give the water a momentum in the opposite direction, the hand manipulates
the water and puts lift on the hand and momentum on the water in the opposite
direction.