2.11-2.15 ( fluids)

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# 2.11-2.15 ( fluids) - PowerPoint PPT Presentation

2.11-2.15 ( fluids). What happens when you jump into water? The water pushes aside (displaces) to make room for you. All fluids, gases as well as liquids, behave this way when an object is placed in them.

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### 2.11-2.15 ( fluids)

What happens when you jump into water? The water pushes aside (displaces) to make room for you. All fluids, gases as well as liquids, behave this way when an object is placed in them.

• The fluid also pushes back in all directions when a object is placed in it . The upward part of the force exerted by fluids is called buoyancy. Buoyancy is a property of all fluids. Buoyancy is the tendency for materials to rise or float in a fluid.

Buoyancy is not the only force acting on a object in a fluid. The force of gravity (weight) also acts on an object. The particles of a fluid exert a force in a direction opposite to the force of gravity, which pulls down toward the centre of the earth. Buoyant force is measured in Newtons (N). Without gravity there would be no buoyancy. We say that the object has positive buoyancy, negative buoyancy, or neutral buoyancy according to whether it rises, sinks, or remains level in a fluid.

Buoyancy and Gravity
• If you want to know whether an object will sink or float, you need to consider all the forces that are acting on the object.
• An object will rise in a fluid when:

_ the density of the object is less than the density of the fluid

_ the buoyant force on the object is greater than the force of gravity on the object

An object will sink in a fluid when:

• - the density of the object is greater than the density of the fluid
• - the buoyant force on the object is less than the force of gravity on the object

An object will float in a fluid when:

• - the density of the object is equal to the density of the fluid
• - the buoyant force on the object is equal to the force of gravity on the object

Design has a lot to do with whether an object will sink or float- if the weight of the object is spread over a large enough area, water can support objects that have densities greater than water.

• The average density of an object is the total mass of all substances that are enclosed in the object, divided by the total volume. Average density is useful, because it enables objects that would otherwise sink- such as large ships (Titanic) to float.
Archimedes Principle
• More than two thousand years ago, a Greek Mathematician and inventor named Archimedes discovered an important principle about buoyancy. Archimedes’ principle states that the buoyant force on an object is equal to the weight of the fluid displaced by the object.
• Archimedes used his own body (which displaced water in the bathtub) to prove the King’s goldsmith was cheating the king.

The buoyant force of a liquid does not depend on physical state, but rather on density. The relationship between buoyancy and density is the basis for the hydrometer, which is an instrument designed to measure density directly. The higher the hydrometer floats in the liquid being tested, the higher the density is (of the liquid). Hydrometers are used widely in the food and beverages industries.

Ballast
• Ballast is any material carried on ships, submarines, hot air balloons, or dirigibles (air ships) that acts as weight and alters buoyancy. The ballast helps the vessels to be stable and to travel at the appropriate level in the fluid. Tanks of the water often provide the ballast for ships and submarines.
Definitions
• 1) Archimedes’ principle
• 2) Ballast
• 3) Buoyancy
• 4) Negative buoyancy
• 5) Neutral buoyancy
• 6) Plimsoll lines
• 7) Positive buoyancy