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You can observe displacement by filling a glass to its brim with water. If you push a ping-pong ball into the glass, the water runs over the side. The volume or amount of water that overflows the glass is the volume that has been displaced. If you weigh the water that overflows and the ping-pong ball, you find that the water weighs more than the ping-pong ball. If an object displaces an amount of water weighing more than it does, it floats. If an object displaces an amount of water weighing less than it does, it sinks. If an object displaces an amount of water weighing the same as it does, it hovers. States of Buoyancy Buoyancy is always an upward force tending to cause you to float, rise, hover, or sink less quickly (figure 4-7). As a diver, you control your buoyancy primarily by the amount of weight you wear and the amount of air you have in your buoyancy compensator (BC) or dry suit. Positive If an object floats, it means the object displaces an amount of water that weighs more than the object does. In other words, the object is less dense than the water. You want to have enough buoyancy to float when you are at the surface before and after you dive (figure 4-8). Neutral If an object is neutral, it hovers. This means the object displaces an amount of water that weighs the NAUI Scuba Diver 98 Diving Science FIGURE 4-7. BUOYANCY IS ALWAYS AN UPWARD FORCE TENDING TO CAUSE YOU TO FLOAT, RISE, HOVER, OR SINK LESS QUICKLY. FIGURE 4-8. YOU WANT TO FLOAT, OR BE BUOYANT WHEN YOU ARE AT THE SURFACE. FIGURE 4-9. YOU WANT TO BE NEUTRALLY BUOYANT DURING YOUR DIVE.


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