WATER AND THE AQUATIC ENVIRONMENT

DENSITY AND FLOTATION Density:

A 50g cube of Styrofoam and a 50g cube of brass were placed in a basin of water. Even though they both have the same mass and weight the Styrofoam floats but the brass weight sinks. This is because of differences in their densities relative to that of water.

The density of a material is defined as its mass per unit volume. An object floats in a fluid when it is less dense than the fluid, and it sinks if it is denser than the fluid. Styrofoam is about 30 times less dense than water, so it floats. On the other hand, brass has a density of about 8 times that of water and so it sinks. The density of pure water is 1 g/cm3 or 1000 kg/m3. This means that any object that has a density of more than 1 g/cm3 will sink in water, while any object with a density that is less than 1 g/cm3 will float. To calculate the density of an object we need to know its mass and its volume.

Density = Mass

Volume Archimedes Principle and flotation: When an object is held underwater, it displaces some water particles. It displaces an amount of water that has the same total volume as itself. This creates an upward force on the object equal to the weight of the water displaced.

Archimedes principle (also known as the law of buoyancy) states that an object submerged in a fluid experiences an upwards buoyant force (upthrust) equal to the weight of the fluid that is displaced. An object floats if the upthrust is equal or greater than its weight. It sinks if the upthrust is smaller than the weight of the object.

The shape of an object is important in determining whether it floats or sinks. If a lump of plasticine is dropped in a bowl of water it will sink. However, if the same lump of plasticine was flattened out in a shape similar to a boat it will float. This is because the plasticine, when flattened out in the shape of a boat, is able to displace a larger volume of water than when it was in a lump. This means it becomes more buoyant and it floats. The magnitude of the upthrust acting on an object is related to the density of the fluid that it is immersed in.

Ø The density of water changes with temperature, and reaches a maximum at 4°C. Ø The density of water is also affected by its purity. Water that contains dissolved

salts has a higher density than pure water. For example seawater is denser than fresh water and objects that float in seawater may not float in fresh water.

A ship that floats on the cold North Atlantic Ocean might sink once it reaches the tropics. Plimsoll lines are thus painted on the hulls of ships to ensure that they are not loaded beyond their buoyancy limits. The Plimsoll line, officially known as the International Load Line, indicates the highest mark to which the waterline can reach in different types of water when the ship is loaded.

Water Safety Devices:

• Life jackets are designed to be very light, and yet occupy large volumes. When a life jacket is worn, both the person and the life jacket can be considered as a single body. The weight increase is very slight, while the upthrust increases significantly. This helps to keep the person afloat, preventing him from drowning. Devices like life jackets are known as personal flotation devices or life preservers.

• Safety rings are throwable life preservers that are similar to the inflatable rubber rings that are used when learning to swim. These devices also work by providing buoyancy to people using them.

Hazards of scuba diving: To stay underwater for long periods of time, divers have to use self contained underwater breathing apparatus (SCUBA). The solubility of gases in water increases at high pressures. Since the pressure increases as a diver goes down to greater depths, the solubility of gases in the diver’s blood and tissue fluids also increases.

Ø When the amount of dissolved nitrogen in the tissues exceeds a certain threshold, divers might develop a condition known as nitrogen narcosis. This usually happens at depths of 30m or deeper. The effects on the diver include loss of decision making ability, poor coordination and judgement. The treatment of nitrogen narcosis involves ascending to a lower depth, and breathing oxygen to purge the nitrogen from the body.

Ø At high pressures even oxygen becomes toxic. Oxygen poisoning can lead to nausea, dizziness, convulsions and death. There is no known treatment for oxygen poisoning. To reduce the risk of oxygen poisoning, divers use special tanks in which a proportion of the oxygen and nitrogen in air is replaced with non-toxic helium.

Ø When a diver ascends, the air in the blood becomes less soluble and is released as gas bubbles. If the diver resurfaces too quickly, the gas bubbles that are formed can block off blood circulation, leading to spasms, paralysis and even death. This is a form of decompression sickness known as the bends. Also, if the divers ascent is too rapid, and the rapidly expanding air in the lungs is not breathed out, the divers lungs may rupture. To prevent this painful condition from developing, divers rise slowly and make intermittent stops on their ascents. These decompression stops allow air bubbles that are formed to be released naturally through the lungs. A diver that makes a quick ascent due to lack of air in his tank, has to be taken to a recompression chamber. The pressure in the chamber is increased to re-dissolve the bubbles in the body. The pressure is then lowered gradually so that the dissolved air can be released safely.

MOTION IN FLUIDS A fluid is any substance which flows. Fluids include all gases, vapours and liquids. Resistance to motion in fluids: Have you ever tried running while you are in a swimming pool? If you have you will find that it is much easier to run on land than in water. This is because you experience a much larger drag (resistance to motion) when moving in water than in air. When objects move through fluids, they collide with fluid particles, and this tends to slow them down. The amount of drag experienced by an object depends on several factors, including the velocity of the object, the nature of the fluid and the shape of the object.

Ø Drag always acts in the direction opposite to the motion of the object. A motionless object will experience no drag. The faster an object moves the more drag it will experience.

Ø The density and viscosity of the fluid affect the amount of drag that an object moving in the fluid experiences. The viscosity of a fluid is a measure of its resistance to flow. Honey and molasses are examples of highly viscous liquids. The higher the density and viscosity the more drag will be experienced.

Ø The shape of an object also affects the amount of drag. Drag is higher when the cross-sectional area that is perpendicular to the motion is larger. Streamlining is the shaping of an object to minimize the drag it experiences. A moving object has to spend energy to over come drag. When the drag is larger, more energy is needed to maintain the motion of the object.

Vehicles and sportswear are designed to be streamlined so as to reduce energy losses. Streamlined objects usually have well rounded fronts and curved bodies. These features help the fluid to flow smoothly around the object, reducing drag. When drag is a good thing: Although drag is undesirable in most cases, it sometimes made use of in sports too. A badminton shuttlecock has a rounded head made of cork or plastic, and a skirt made of plastic. Due to their shapes, the skirt experiences more drag than the head and is slowed down more. As a result, regardless of its initial position, a shuttlecock that flies through the air always turns to fly head first. You have probably seen football players skillfully bending a free kick around a wall of players. The ball curves when the player gives it a spin as it is kicked. As a spinning ball flies through the air, the side that is moving against the direction of the air experiences a larger drag. This causes the ball to deflect to the opposite side due to pressure differences.