In swimming, Newton's First Law explains that:

• A. Inertia has no effect on energy requirements in water.
• B. Extra energy is needed to start, stop, or change direction in water due to inertia.
• C. Water movement eliminates inertia.
• D. Momentum in water is always conserved without any energy changes.

Answer: (B)

Newton's First Law describes inertia: a body tends to keep doing what it is doing unless an external force acts. In water, your body has inertia, and the water around you provides resistance. To start moving from rest, you must push water backward, and to stop or change direction you must alter that interaction, applying forces to water that create the needed change in momentum. Because water is dense and creates drag, more energy is required to overcome this resistance and to alter your motion. Water movement does not erase inertia, and momentum isn’t freely conserved without energy changes because the swimmer-water system constantly exchanges forces and energy during propulsive actions. This is why extra energy is needed to start, stop, or change direction in water.