Movement in Water
Animals moving through water face a different set of challenges from animals moving through air. One big difference is that water is more viscous, or sticky, than air. This means that more force is needed to move something through water than through air. You can see this yourself by trying to run in a swimming pool. You have to put more force into each step. The same viscosity that makes it harder to move forward also makes it harder to fall. Animals in water don't fall as fast as animals in air, because, although the force of gravity hasn't changed, the viscosity (stickiness) of the environment has. So in water, animals don't "feel" gravity as much as animals on land. This aspect can actually be beneficial. The video to the right demonstrates viscosity with water and glycerin.
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Let's look at the forces that act on animals in the water. Consider the swimmer in the free-body diagram to the left. This shows the four main forces acting on animals moving through the water. Gravity, although less of an issue in water, still pulls the swimmer downward. But in water, gravity is counteracted by buoyancy, the force pulling the animal up. Buoyancy is very evident in inflatable beach balls. Beach balls float on the surface of water because the buoyant force of the beach ball is stronger than the force of gravity pulling it down. Fish can control their buoyancy with their swim bladder, a sack of air in their bodies. They can add or subtract air from this swim bladder to keep them buoyant enough to counteract gravity. Thrust is the force acting in the direction of the swimmer's movement. This is the force that will propel the swimmer forward. Drag is the force acting in the opposite direction. If the swimmer is swimming forward, drag will act to slow the swimmer.
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How these forces act on an animal has a lot to do with the shape of the animal. To understand this, imagine you are jumping from a diving board into a pool and recording how deep into the pool you go. You do three jumps: a pencil dive (where your hands are to your sides and you jump in feet first), a cannon ball (where you grab your knees in midair and hold them close to your body throughout the jump), and a belly flop (where you jump in face first with your hands and legs spread out flat). Which jump will make you go the deepest into the pool? You would go the deepest on the pencil dive, medium deep with the cannonball, and you'd stay pretty shallow with the belly flop. The only thing that changed between these three jumps was the shape of your body as it was going into the water. But this difference in shape made a big difference in how far you went into the pool. Note that you used the same amount of force on each jump, the force of gravity.
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Animals use shape to their advantage to help them move through the water (or let water move over them, if they are attached to the bottom). Take a look at all of the fish and whales on the left. What do you notice about their shape? They all have a very similar shape, called fusiform, or cigar-shaped. This shape helps relatively large animals, like fish and whales, to move through the water with very little drag. This is called being hydrodynamic. Without as much drag, these animals can move through the water while expending very little energy.
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Animals also use their shape to brake (by becoming less hyrodynamic, like by holding their fins out on both sides) and turn (by being more hydrodynamic on one side than the other, like by holding out one fin to the side and leaving the other flat). Look at this parrot fish on the right. It is using its pectoral fins (the fins on the sides) to steer its body so it can eat the smaller fish in front of it. If this fish kept its fins to the sides of its body, it would be more hydrodynamic, allowing it to move forward faster. By holding out its fins the way it is, this fish is slowing itself down and steering. |
Animals moving in water also have to contend with the water around them moving in different ways. Turbulence is water moving unsteadily. Imagine shaking a bottle of water. The motion is creating turbulence in the bottle. Currents are steady flowing of water in one direction, like in a river. Eddies are swirling water. When you move your arm through a pool of water, it leaves eddies behind it. Tides are the rising and falling of water on the shoreline each day. Animals have to contend with each of these different ways that water moves around them. Imagine trying to walk in a straight line on a trampoline while someone else is bouncing on that same trampoline. It's pretty hard, right? Animals who move in water deal with that every day. |
Even in a constant flow of water, like a stream, the speed of the flowing water is not the same everywhere. Because water is viscous, it sticks to solid objects it touches. So when there is a surface submerged in water (like the rocks on the bottom of a river), water particles near the surface stick to it and don't move as quickly as water particles farther away. This stickiness results in a boundary layer, which is a gradient of flow speeds that get slower the closer it is to the solid object. This all happens very close to the surface of the object, within a few centimeters. But it can have very real impacts on small animals in water. A small tadpole hanging out at the bottom of a river will feel almost no flow. But as soon as that tadpole moves up away from the bottom, the tadpole will feel a much faster flow of water. Where the tadpole is in this boundary layer will have a significant effect on how it moves.
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The size of an animal also effects how it feels the viscosity of the water. A relatively large animal, like a tuna or a sea lion, will feel the viscosity of the water like you do when you go swimming. The water sticks to you in droplets when you get out, but it's only a minor impediment to you swimming. But to very small animals, like the plankton to the left, the viscosity of water is a big factor. To very small creatures like these, the water feels very sticky, like how molasses or wet clay, feels to us. Although the force of friction created by the water's viscosity acts on all sizes of animals the same, larger animals aren't effected by it as much. It's the same idea as swatting a fly. If you swat a fly with a flyswatter, you can squish the fly. But if you swat yourself with the same amount of force, at most you will have a stinging sensation, but you won't be squashed. The force doesn't have as strong of an effect on you because you are a much larger size. An interaction between size and viscosity (and speed) is calculated into one variable called the Reynold's number. |
We've learned a bit about the challenges animals face moving in water. Let's turn now to how they actually do it. Just like with moving on land, different animals move in different ways in water. Let's start with humans. Whether you do a doggy paddle or swim freestyle, you probably swim by kicking your legs and swinging your arms through the water. When you kick your legs, you are pushing vortices of water below you, keeping you at the surface. When you move your arm back, you are pushing water behind you, therefore moving you forward into the space the water once occupied. The video to the right shows how biomechanics research can improve swimmer's performance.
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Animals move in water in various ways. Many fish undulate their tails back and forth to move forward. When they do this, they shed vortices, circulating water, behind them with every beat. These vortices accelerate the water behind them. This acceleration of the water behind them, accelerates the fish forward. If you want a more detailed explanation of how that works, check out this great website.
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Drag based thrust is how a lot of legged animals swim in water, like humans, sea lions, and dogs. Drag-based thrust is the same thing a paddle uses to propel a canoe. When you drag a paddle through the water, it creates a drag force on the paddle (remember, drag is the force pushing in the direction that the water is flowing). But while this drag force is acting on the paddle, it's creating a force in the opposite direction (thrust) with respect to the whole canoe. This same thing happens when you doggy paddle in the pool. Your hand goes backwards and creates a drag on the hand. But at the same time, it creates a thrust on your whole body causing your body to move forward. The video on the right explains how this works with Olympian, Missy Franklin.
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Lastly, let's consider how boats move through water. We've learned that in order to move forward, the boat needs to have enough thrust to overcome it's drag. And in order not to sink, it needs to be buoyant. Most boats are buoyant enough by containing air, or by manipulating their shape to be more stable. But what about thrust? We've learned that paddling boats use drag-based thrust to move forward. But what about power boats? Many motorized boats use propellers to generate thrust. They pull water through them, similarly to a screw going through a wall. Animals typically don't have a true propeller for the same reason they don't have wheels; muscles and tendons don't allow for free rotation around an axle.
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Now that we've covered land and water, let's consider how animals move through the air in Part Three.