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1. Submarine:

A submarine has a large ballast tank, which is used to control its position and depth from the surface of the sea.

A submarine submerges by letting water into the ballast tank so that its weight becomes greater than the buoyant force.

Conversely, it floats by reducing water in the ballast tank.-thus its weight is less than the buoyant force

2. Hot-air balloon

The atmosphere is filled with air that exerts buoyant force on any object.

A hot air balloon rises and floats due to the buoyant force (when the surrounding air is greater than its weight). It descends when the balloon's weight is higher than the buoyant force. It becomes stationary when the weight equals the buoyant force.

The weight of the Hot-air balloon can be controlled by varying the quantity of hot air in the balloon.

3. Hydrometer

A hydrometer is an instrument to measure the relative density of liquids.

It consists of a tube with a bulb at one end. Lead shots are placed in the bulb to weigh it down and enable the hydrometer to float vertically in the liquid.

In a liquid of lesser density, a greater volume of liquid must be displaced for the buoyant force to equal to the weight of the hydrometer so it sinks lower.

Hydrometer floats higher in a liquid of higher density.

Density is measured in the unit of g cm-3.

4. Ship

A ship floats on the surface of the sea because the volume of water displaced by the ship is enough to have a weight equal to the weight of the ship.

A ship is constructed in a way so that the shape is hollow, to make the overall density of the ship lesser than the sea water. Therefore, the buoyant force acting on the ship is large enough to support its weight.

The density of sea water varies with location. The PLIMSOLL LINE marked on the body of the ship acts as a guideline to ensure that the ship is loaded within the safety limit.

A ship submerge lower in fresh water as fresh water density is lesser than sea water. Ships will float higher in cold water as cold water has a relatively higher density than warm water.

5. Fishes
Certain group of fishes uses Archimedes’ principles to go up and down the water.
To go up to the surface, the fishes will fill its swim bladder (air sacs) with gases (clever isn't it?).
The gases diffuse from its own body to the bladder and thus making its body lighter. This enables the fishes to go up.
To go down, the fishes will empty their bladder, this increases its density and therefore the fish will sink.
6. FLIP – Floating instrument platform.This is a research ship that does research on waves in deep water. It can turn horizontally or vertically. When water is pumped into stern tanks, the ship will flip vertically.
The principle that is used in FLIP is almost similar with the submarines. Both ships pump water in or out of tank to rise or sink.

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