WSI - Water Safety Instructor Hydrodynamics and Basic Skills Questions and Answers — Questions and Answers
Question 1: A swimmer's legs tend to sink, causing an increase in drag. This is primarily due to the relationship between which two hydrodynamic principles?
- Propulsion and Form Drag
- Center of Buoyancy and Center of Gravity (Correct answer)
- Wave Drag and Friction
- Bernoulli's Principle and Lift
Correct answer: Center of Buoyancy and Center of Gravity
The center of gravity (mass) in a human is typically lower, around the hips, while the center of buoyancy is higher, around the chest, due to air in the lungs. This separation creates a torque that causes the denser legs and hips to sink, rotating the body out of a streamlined horizontal position and increasing drag.
Question 2: A Water Safety Instructor is teaching a student the importance of a streamlined push-off from the wall. Which type of drag is most significantly reduced by maintaining a tight, streamlined position during a glide?
- Wave Drag
- Frictional Drag
- Form Drag (Correct answer)
- Propulsive Drag
Correct answer: Form Drag
Form drag (or pressure drag) is the resistance created by a swimmer's shape and cross-sectional surface area as they move through the water. A tight streamline minimizes this surface area, allowing the swimmer to cut through the water more efficiently, thus reducing form drag. While other drags exist, form drag is the most profoundly impacted by body position and shape.
Question 3: Which of the following best describes the application of Newton's Third Law of Motion to swimming?
- A swimmer will remain gliding at a constant speed unless slowed by the water's resistance.
- The greater the force a swimmer applies to the water, the faster they will accelerate, assuming their mass is constant.
- A swimmer with more muscle mass will require more force to achieve the same acceleration as a smaller swimmer.
- As a swimmer's hands and arms push the water backward, the water pushes the swimmer forward. (Correct answer)
Correct answer: As a swimmer's hands and arms push the water backward, the water pushes the swimmer forward.
Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. In swimming, the 'action' is the swimmer pushing water backward with their hands, arms, and feet. The 'opposite reaction' is the water pushing the swimmer's body forward, creating propulsion.
Question 4: During a lesson on treading water, a student is struggling using the flutter kick and is tiring quickly. Which of the following kicks would be most effective and efficient for the student to maintain a vertical position?
- Dolphin Kick
- Scissor Kick
- Rotary (Eggbeater) Kick (Correct answer)
- Inverted Frog Kick
Correct answer: Rotary (Eggbeater) Kick
The rotary, or 'eggbeater', kick is the most efficient and effective method for treading water. It provides continuous upward support because one leg is always in the power phase of the kick. The flutter kick is generally inefficient for sustained treading.
Question 5: A swimmer is performing the front crawl with their head lifted too high. What is the most likely hydrodynamic consequence of this error?
- It generates greater propulsive lift.
- It causes the hips and legs to drop, increasing form drag. (Correct answer)
- It decreases wave drag around the head.
- It improves the body's center of buoyancy.
Correct answer: It causes the hips and legs to drop, increasing form drag.
Lifting the head high while swimming the front crawl disrupts the body's horizontal alignment. This action causes the hips and legs to drop lower in the water, which significantly increases the body's frontal surface area and, consequently, form drag. This makes swimming much less efficient.
Question 6: According to Archimedes' principle, the upward buoyant force that is exerted on a body immersed in water is equal to the:
- Mass of the body.
- Volume of the water.
- Weight of the water the body displaces. (Correct answer)
- Density of the body.
Correct answer: Weight of the water the body displaces.
Archimedes' principle states that the buoyant force on a submerged or floating object is equal to the weight of the fluid displaced by the object. This principle explains why people float and is a fundamental concept in hydrodynamics.
WSI - Water Safety Instructor Hydrodynamics and Basic Skills Questions and Answers
A swimmer's legs tend to sink, causing an increase in drag.
This is primarily due to the relationship between which two hydrodynamic principles?