While land animals often have awkward, non-aerodynamic shapes, sea animals are usually pretty streamlined. That difference is explained by the enormously greater forces of water resistance as compared to air resistance. In a nutshell, a non-hydrodynamic sea animal had better have a thick shell, taste lousy, or expect to be eaten.

Dolphins are a case in point. They have evolved wonderfully streamlined and hydrodynamically efficient shapes. The interactions between a dolphin and the water around it offer many opportunities to examine the physics of fluids and motion.

1. While a dolphin is a mammal and all mammals have hair, what few whiskers the dolphin develops during gestation are lost around the time of birth. The result is that dolphins have as little surface area as possible for their basic size and shape. Why does minimizing its surface area tend to make the dolphin more energy efficient while swimming through the water? (Note: ignore issues related to golf ball dimples or tennis ball fuzz, which are not relevant to this question.)

2. In addition to having a small surface area, the dolphin has a remarkably streamlined shape. Its nose and head curve out and back, with no sharp edges or forward-projecting appendages. The dolphins body reaches its maximum girth about a third of the way from its nose to tail and then the dolphin's body tapers gradually to a narrow tip that finally ends in its broad, flat tail fluke. Because of this carefully developed shape, the dolphin creates almost no turbulent wake when it moves through the water. Why does the near absence of a wake make the dolphin more energy efficient when it swims?

3. Imagine that you are riding along through the water with the dolphin and watching the water pass by the two of you. That water collides with the dolphin's rounded nose and spreads outward, away from the dolphin's skin. (A) Compare the water pressure on the tip of the dolphin's nose to the pressure in the freely flowing stream of water nearby. (B) Compare the water's speed on the tip of the dolphin's nose with the water's speed in the freely flowing stream.

4. As you continue to ride along with the dolphin, you observe the water flowing around its sides. This water bends inward, toward the dolphin's skin, as it follows the dolphin's inward curving sides. (A) Compare the water pressure in the water passing very close to the dolphin's side to the water pressure in the nearby freely flowing stream. (B) Compare the speed of the water passing very close to the dolphin's side (but not in the boundary layer) to the speed of the water in the freely flowing stream.

5. An object as large as a dolphin (about 3 meters) and traveling as fast as a dolphin (about 10 mph) should create turbulence in the water. (A) Give a rough estimate of the Reynolds number describing the water flow around a dolphin cruising at its typical speed. (B) What type of flow does that value predict?

6. As part of its play, a dolphin enjoys leaping through the air. On occasion, it will squirt water out of its mouth while above the surface. The water is barely moving while it's in the dolphin's mouth, but it acquires a high speed as it shoots through the dolphin's lips on the way out of its mouth. What happens to the water's (A) pressure, (B) speed, (C) pressure potential energy, and (D) kinetic energy as that water travels toward the dolphin's lips.

7. Whether it is buoyant or not, the dolphin can control its depth in the water by adjusting the angles of its body and fins as the water passes by. How does the horizontally moving dolphin obtain the vertical forces that it needs to either (A) lift itself upward against gravity or (B) sink itself downward against its own buoyancy?

8. During a skyward leap, the dolphin rises completely out of the water for a fraction of a second. There is even a time during which the dolphin is rising straight upward while not touching the water at all. During this period of upward motion, is there any upward force acting on the airborne dolphin and, if so, what is that upward force? (Neglect any buoyant effects due to the air.)