Chapter 3- Diving Physics Diving Physics 79 to hover in the ocean. The amount of weight she must wear in the ocean will be the difference between the required total and her basic 80 kilograms (without any ballast weight). Method 1: Find the diver’s volume. Volume = Weight (mass) = 88 kg = 88 L Density 1 kg/L Find the buoyant force of seawater, which is the weight of the seawater displaced: Buoyant force = Volume x Density of seawater = 88 L x 1.025 kg/L = 90.2 kg. The weight the diver must wear in the ocean is the difference between the buoyant force of the seawater and her weight with no weight system in place. 90.2 kg – 80 kg = 10.2 kg. Because 0.5 kg weights are available, 10.5 kg is a good, practical answer. The fine trim of the diver’s buoyancy will be provided by slight adjustments with her BC and her lung volume as she breathes. The second method is to bypass calculating the volume of the diver. Multiply her required total weight for fresh water by the ratio of the density of seawater to the density of fresh water. The answer will be the total weight she must achieve to be able to hover at her safety stop in the ocean. Method 2: The diver’s total weight for seawater is her total freshwater diving weight times the ratio by which seawater is denser than freshwater (1.025 kg/L) : ( 1 kg/L ) 88 kg x 1.025 = 90.2 kg U.S./Imperial A solid block of material measuring 5 feet long x 4 feet wide x 1 foot high weighs 960 pounds. If placed in the ocean, will the object sink or float? If the object sinks, what will be its in-water weight underwater. If the object floats, how much of the object will be submerged? Answer: The dry weight of the block (960 lbs) and the density of seawater (64 lbs/ft3) are known. The buoyant force (weight of the water displaced if the block is totally submerged) is the density of the water times the volume of the block, which is: 5 ft x 4 ft x 1 ft = 20 ft3 The weight of an equal volume of seawater is: 64 lbs/ft3 x 20 ft3 = 1,280 lbs The object weighs less than an equal volume of water. It will float. It will sink in the ocean until it has displaced 960 pounds of water. To find the submerged portion of the object, divide 960 lbs/20 ft3 (the density of the block) by 1,280 lbs/20 ft3 (the density of the seawater): 960 / 1,280 = 0.75 Seventy five percent (or 9 inches) of the block will be underwater. 3. A fully equipped diver, including his weight belt as well as all of his other equipment weighs 260 pounds, and he is able to hover comfortably (with no air in his BC) during his decompression stop in the ocean. How many pounds should he remove from his ballast weight to dive in fresh water? Answer: At the end of his dive, the diver’s density is equal to seawater density. In a fresh water dive, the water will provide less buoyant force. As before, there are two ways to solve this problem. Either compute the volume of the diver and use this value to compute the buoyant force provided by fresh water, or multiply the diver’s total weight for ocean diving by the ratio of the density of fresh water to the density of seawater. Method 1: find the diver’s volume. Volume = Weight (mass) = 260 lbs = 4.063 ft3 Density 64 lbs/ft3 Find the buoyant force of fresh water. The buoyant force is the weight of the water displaced, which is: 4.063 ft3 x 62.4 lbs/ft3 = 253.5 lbs. The difference between the diver’s total weight for ocean diving and his required weight for fresh water diving is the amount of weight he must remove: 260 lbs – 253.5 lbs = 6.5 lbs, or about 6 pounds. Method 2: The total fresh water diving weight is the total seawater diving weight times the ratio by which fresh water is less dense than sea water (62.4 lbs/ft3) : (64 lbs/ft3) 260 lbs x 62.4/64 = 253.5 lbs
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