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Principle of Operation of Vehicles in Water

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Vehicles in water are moved by a) energy from humans such as boats b) wind energy such as boats c) energy from the engine such as ships, hydrofoils and hovercrafts.

 

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Boats a) boats are moved with oars b) when water is pushed backwards by oars in a boat, a backward momentum is produced c) a forward momentum of equal magnitude to the backward momentum is produced, thus moving the boat forwards.

 

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Sailing boats/ yachts a) sailing boats and yachts move by using wind power. These boats have one or more sails with large surface areas b) when the wind blows, the movement of the wind is blocked by the sails. The momentum of the wind is transferred to the sails, causing the boat to move forward.   

 

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Ships a) large ships use diesel engines or nuclear energy to move their propellers located at the end of the ship b) large amounts of water is pushed backwards very quickly by the propellers, creating a large backward momentum c) a forward momentum that is of equal magnitude to the backward momentum is produced, causing the ship to move forward.

 

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Hydrofoils a) a hydrofoil consists of underwater foils attached to the front and rear of its hull b) when a hydrofoil moves slowly; it behaves like any other boat. Energy from the diesel engine rotates the propeller and pushes water backwards. The backward momentum of water produces a forward momentum that pushes the hydrofoil forward c) When a hydrofoil accelerates to a higher speed, the flow of water over and around the foil produces an upward force called lift or upthrust d) this will raise the hydrofoil out of the water. Eventually, most of the hull will be out of the water. Friction with water is reduced and the hydrofoil can travel much faster.

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Hovercrafts a) a hovercraft is a vehicle that floats on top of a cushion of air while moving. It can skim across land or water b) a central fan draws in air from the atmosphere and forces it between the hull and the outer skirt, forming an air cushion c) the air cushion is also used to produce propulsion. The propulsion is produced by the rotation of propellers or through propulsive jets e) the backward momentum of water or air produces a forward thrust that propels the hovercraft.

The Shape of Vehicles that Facilitate Motion in Water

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An object moving through the water needs to overcome the drag against it. The larger the surface area of the object that comes into contacts with water, the larger the drag when moving through water. A streamlined-shaped object can travel through water with the least water resistance or drag.

The smooth sides and rounded outline of the shape do not disturb the flow of water and do not cause turbulence behind the object. Turbulence causes drag in a vehicle. The hulls of vehicles that are immersed in water are streamlined so that the vehicles can sail faster through the water. The center of gravity of the vehicles in water must be below the water level so that the vehicle is stable and balanced when moving on water.

Archimedes’ Principle

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The pressure in a liquid increases with depth. When an object immerses in water a) the object does not move sideways because the pressure exerted on both sides of the object is equal. The depth of both sides of the object is equal b) the bottom surface of the object is deeper than that of the top surface. The upward pressure exerted on the bottom surface of the object is greater than the downward pressure exerted on the top surface. The difference in pressure exerted on the object produces a net upward force called the buoyant force or upthrust.

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The Archimedes’ Principle states that an object, whether completely or partially immersed in a fluid (liquid or gas), is acted on by an upthrust which is equal to the weight of the displaced fluid. The principle of floatation states that the weight of an object floating on the surface of a liquid is equal to the weight of the liquid displaced by the object. There are two forces acting on a submerged object. One force is the upward buoyant force and the other force is the downward gravitational force or the weight of the object. An object will a) sink if the weight of the object is greater than the buoyant force b) move upwards if the buoyant force is greater than its weight c) remain stationary in a liquid or float on the surface of the liquid if the buoyant force is equal to its weight.

Applications of the Archimedes Principle

Ships

 

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A ship floats on the surface of sea water if the upward buoyancy force that it receives is great enough to overcome the downward force of its weight. A steel ship is hollow inside so that the overall density of the ship is less than that of sea water. The level a ship sinks in the sea depends on the buoyant force that acts on it.

The deeper a ship sinks in the sea, the larger the volume of sea water that is displaced. Hence the buoyant force increases. The density of sea water varies with location. This will vary the level that the ship sinks in the sea. The denser the sea water, the larger the buoyant force. The ship will submerge deeper in fresh water than in sea water. The density of sea water varies with the region it is in and the season it sails. Therefore, most ships are marked with the Plimsoll line. The Plimsoll line is painted on ships to show the depth a ship should travel at.

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Submarines

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A submarine has a large ballast tank used to control the position and depth of the submarine from the surface of the sea. To sink a submarine: a) the ballast tank is filled with water b) this increases the density and weight of the submarine c) the submarine sinks when its weight is larger than the buoyant force.

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To float a submarine: a) water is forced out from the ballast tank with compressed air b) this decreases the density and weight of the submarine c) the buoyant force is larger than the weight of the submarine, causing the submarine to float. If the density of the submarine is the same as the density of the sea water, the depth of the submarine is maintained at one position. A submarine moves forward with the help of a propeller.