Water Safety Instructor (WSI) Practice Exam

In the context of the law of inertia, changing direction during movement requires an external force. The law of inertia, which is derived from Newton's First Law of Motion, states that an object at rest will remain at rest, and an object in motion will remain in motion with the same speed and in the same direction unless acted upon by an external force.

When it comes to swimming or any form of movement, if a swimmer wants to change direction, they must apply a force that alters their current state of motion. This can be achieved through various actions, like kicking, pulling with the arms, or changing the orientation of the body.

In contrast, the other options do not directly facilitate a change of direction according to the law of inertia. An increase in water temperature might affect buoyancy or resistance but does not provide a mechanism for changing direction. Momentum from previous strokes helps maintain speed but does not override the inertia of the swimmer's body to change direction. Lastly, breathing technique is vital for maintaining stamina in water but does not influence the physical laws governing motion. Thus, an external force is indeed necessary for making directional changes during swimming or any other movement.