Water is a 1000 times more dense than air, so when the body moves forward through it, the water resists its movement with a force substantially greater than the resistance of air. That force is resistive drag. Swimmers will accelerate forward so long as the propulsive forces they apply are greater than the resistive drag forces holding them back. They encounter resistive drag as they move forward because they must actually push streams of water molecules out of their way to open a hole in the water for the body to pass through. The resistive drag is directly proportional to the turbulence they create as they swim down the pool and is determined by four factors:
- the space swimmers take up in the water,
- the shape they present to the water,
- limb movements that push water forward instead of backward, and
- the friction between the body and the streams of water that come in contact with it.