Guide Page

Overview of the topic

It is a matter of common experience that bodies appear lighter when immersed in water or any other liquid. While bathing we notice that the mug of water suddenly appears heavier as soon as it comes above the water surface. Similarly, when a fish is pulled out of water, it appears to be heavier in air than inside the water. Now let us see why it is so.

Objects appear to be less heavy in water or in any liquid because the liquid or water exerts an upward force on the objects immersed in it. Now by performing an experiment let us find out whether there is an apparent loss of weight when immersed in water.

Archimedes’ Principle: Buoyancy

Some objects, when placed in water, float, while others sink, and still others neither float nor sink. This is a function of buoyancy. We call objects that float, positively buoyant. Objects that sink are called negatively buoyant. We refer to object that neither float nor sink as neutrally buoyant.

The idea of buoyancy was summed up by Archimedes, a Greek mathematician, in what is known as Archimedes Principle: Any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.

From this principle, we can see that whether an object floats or sinks, is based on not only its weight, but also the amount of water it displaces. That is why a very heavy ocean liner can float. It displaces a large amount of water.

Archimedes principle works for any fluid, but as divers we are mainly concerned with two different fluids: fresh water, and salt water. We need to think of fresh water and salt water as two different fluids because equal volumes of fresh water and salt water do not weigh the same. For example, a cubic foot of fresh water weighs approximately 62.4 lbs, while a cubic foot of salt water weighs approximately 64 lbs. The extra weight is because of the dissolved minerals in salt water.

Archimedes’ principle does not consider the surface tension (capillarity) acting on the body.

Assuming Archimedes’ principle to be reformulated as follows,

apparent immersed weight = weight – weight displaced fluid

then inserted into the quotient of weights, which has been expanded by the mutual volume

density / density of fluid = weight / weight of displaced fluid

yields the formula below. The density of the immersed object relative to the density of the fluid can easily be calculated without measuring any volumes:

density of object / density of fluid = weight / weight – apparent immersed weight

Using the aluminum as our example, it has a specific gravity of 2.8. Water has a specific gravity of 1.0. This means that a cubic centimeter of water would have a mass of 1.0 grams, while aluminum of the same size would have a mass of 2.8 grams. Since the aluminum cube displaces 1 cubic centimeter of water, it has a buoyancy of 1.0 grams. Since buoyancy is a force and not a mass, it must be converted to the proper units, which when multiplied by the acceleration of gravity (980 cm/s²) gives the units of dynes. That is,

(1.0 grams) (980 cm/s²) = 980 grams cm /s² or dynes

So our aluminum cube immersed in water would not ‘weigh’ (2.8 x 980) dynes or 2744 dynes. It would weigh less due to the fact it has a buoyant force of (1 x 980) dynes from the water. So it would weigh (2744-980) dynes or 1764 dynes while immersed in the water.

Components

There are two types of flying machines that allow for lift to overcome gravity. The first type, called the aerodynamic machines such as helicopters and airplanes, rely on thrust and forward speed to produce lift. The second type, aerostatic machines, such as hot air balloons and lighter than air-type craft, rely on the differences in air density for lift.

If a cubic centimeter of aluminum was suspended in a fluid such as water with a very thin and negligible thread, the metal cube would have the fluid exerting pressure on the cube. Try to imagine that if the cube were to disappear, and the fluid would magically replace the cube, then the surrounding water would support this cube that is now containing water, so that the cube of water would be motionless. That is, the forces would be balanced. The cube of water would push out on the surrounding water and the surrounding water would push back on the cube. The fluid would be static, or stationary. Now replace this same cube of water with the original cube of aluminum. The surrounding water would not ‘know’ that the cube has been replaced with another substance. It would still push inward and upward and downward with the same force that it pushed on the cube of water. The sideways forces would be balanced and oppose each other equally, but the upward and downward forces would not be the same. The pressure at the bottom of the cube is greater than the pressure at the top of the cube, because pressure increases with increased depth. The difference between the upward and downward forces acting on the bottom and the top of the cube, respectively, it is called buoyancy.

Did you know?

– Did you know that Archimedes’ principle of Buoyancy is used in designing ships and submarines?

– The lactometers and hydrometers used for measuring the purity of a sample of milk and for determining the density of the liquids are based on this principle.

Why Ships Sink

Ships and boats are made to float on top of the water, but there are quite a few things that can go wrong to turn your boat into a submarine. Taking on water is inevitable — large waves often break over the sides, and tiny leaks are common. This water will usually find its way to the lowest point of a boat — the bilge area. For this reason, boats are equipped with bilge pumps to usher the water back out once it’s reached a certain level. Boats often sink while docked, but unless you’re like Sonny Crockett and you live on your boat, that’s not a life-threatening scenario.

Common reasons a boat might sink at sea are:

Low transom — The transom is the flat vertical surface that forms the rear, or stern end, of the boat. For outboard vessels, the motor is mounted onto the transom. For larger inboard vessels, you’ll find the boat’s name on the transom. The idea is for the transom to be high enough that it won’t take on water. Sometimes, simple design flaws can leave your transom too low. Improper weight distribution can also lower a transom to the point that waves can come over it and flood the deck. To keep this from happening, don’t store all your heavy gear in the stern of the boat. Scuba gear, coolers, fishing equipment and bait should all be distributed evenly along the ship to keep the transom at a safe height. You should also never anchor from the stern side — it could pull the transom down even further.

Missing drain plugs — This one seems like a no-brainer, but boats sink all the time because of missing drain plugs. When a boat travels forward, the entire vessel sits higher on the water than it does at rest, with the front higher than the rear. Water collected from waves or sea spray is allowed to exit the boat through a drain located at the rear of the boat at about deck level. Once you’re traveling forward, the boat tilts up and the water will flow toward the drain and back out. The problem arises when the captain forgets to stop the drain once the boat is at rest with a small, watertight plug. When the boat stops moving, it sinks lower and begins to take on water through the drain. Carry extra drain plugs and try keeping one near the ignition as a reminder.

Cooling system leaks — Boat engines are water cooled, pumping about 30 gallons of water through the system per minute for a 300 horsepower engine. If a hose bursts or isn’t tight enough, this water can collect in the bilge and once again, you could find yourself sinking. Check for corrosion or obvious splits and breaks in the hoses and fittings of the cooling system before you depart. Replace anything that looks suspect, and you should be fine.

Navigation error — Simply put, this means striking an object with your boat. It could be rocks, ice, reefs, logs, or anything else large enough to do damage to the hull, or body, of your boat. The best way to combat this is by being careful. Slow down if you see debris and be especially cautious after storms, which can wash in a great deal of foreign objects. If you see something floating, there’s a good chance there’s more under the surface. If it sounds like you’ve hit something, stop the boat immediately and check outside and below for holes or leaks.

Stick that plug in the drain and click forward to read about what safety equipment you should have on board.

SAFETY PRECAUTIONS INSIDE A SHIP!!

You have to be very thoughtful and considerate to all of the people on board.
It is considered an unwritten law that the captain says what must be done, and his word is not disputable. But there are people and people, so every captain should carefully treat the participants of the crew and passengers.
Beware of boats that are anchored or moored close to the shore when you’re launching. Always aim to pass downwind (or down tide) to them.
Beware of power-crafts (all vessels with engine). Power gives way to sail, but some powerboat owners are not aware of this, go too fast, or have very little knowledge of steering a powerboat. Jet skis are very dangerous, both to their drivers and to the other participants of in the water traffic. However, they are not the only lethal vessels – large power-crafts can be also really treacherous.

Take suitable clothing for all of the people in the crew. Dress up, not down (which means that you should put more clothes than you expect you would need, because if you take less, you may end up in hypothermia).
It means thermal underwear and obligatorily protecting your head from heat loss.
Exposure to water temperatures before 20oC must be treated with seriousness. Very cold water may be experienced when sailing on freshwater lakes and reservoirs, where the temperature can fall really dramatically. You should not be lulled into false sense of security by the sunny weather.

You should always take a paddle in case the wind drops to a flat calm, and a spare rope in case you need the boat to be towed.

Prolonged exposure to cold water causes hypothermia (acute heat loss) and can be dangerous for your health.

You need to dress up to prevent the wind chill and heat loss.

Even if you are in doubt about hypothermia, you should immediately head for the shore.

Beware the sun as well, it can be very harmful.
You should cover all the body parts which are directly exposed to the sun with a sun cream of no less than factor 15, and re-apply it per 2 hours. A secondary effect of the direct sun is dehydration. It is easy to ignore or even not notice the effects of drying, until symptoms become too obvious – they can vary and include perched mouth and a muzzy, tired headache.
The best cure in this case is prevention, to make sure that you drink enough liquid (preferably fresh water).

One of the scariest things is to get lost in a fog. It is not only frightening because you don’t know where the shore is, the more terrible thing is that you don’t know what vessels move around you and at what distance they are.

Fog should be avoided at any costs:

• there is no or almost no wind (so you may stay for a long time in this still)
• it will most probably be clammy, cold and unpleasant
• they say chances of collision are very high
• fog has the persistent habit of going back further in the sea, so when you are able to see clearly, just go to the coast and wait until it clears out completely.

Thunder and lightning

It is important that you take into account that there is a slight chance of your yacht being hit by a lightning, but this happens rarely, and the risks of being hit on land, or die under the wheels of a car are equal if not less.

Learn More!

Center of buoyancy

An object’s center of buoyancy is a point around which the buoyancy forces are balanced. Because the lungs provide a large buoyant volume of air, the center of buoyancy is usually located in the chest. The center of mass is below the center of buoyancy.

When a person of average build tries to float horizontally on the back with arms along the sides of the body, the center of mass is nearly level with the center of buoyancy. Many people have more body weight in their legs and hips because of the high proportion of muscle tissue there, so their center of mass is near the hips. Thus when trying to float in a horizontal position, gravity pulls the hips and legs downward while the buoyant force of the water pushes the chest area (center of buoyancy) upward. The body rotates until the center of mass is directly below the center of buoyancy, resulting in a diagonal body position. At that point, the person should float motionless.

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