section 2. how can a huge ship float? because the force pushing up on the ship opposes the weight...
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Section 2
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How can a huge ship float?• Because the force pushing up on the ship
opposes the weight of the ship pushing down
Supporting force is called : Buoyancy
Buoyancy:• Ability of a fluid (liquid or a gas) to exert an
upward force on an object immersed in it.• If the buoyant force is less than an object’s
weight it sinks.
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Buoyant force on an object is = to the weight of the fluid displaced by the object.
Example: • Place a block of wood in water. At first it
starts to sink. • While it sinks it pushes water out of the way. • Once the weight of the water displaced = the
weight of the block
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Example continued:
• Once the weight of the water displaced = the weight of the block it floats!
If the weight of the water displaced is less than the weight of the block it sinks.
Figure 13
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Would a steel block the same size as a wood block float in water?
They both displace the same volume and weight of water.
Therefore the buoyant force on the blocks is equal.
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What is different?
The volume of the blocks and the volume of water of water displaced have different masses.
If they have different masses they have different densities. Remember D = m/v
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The density of the steel block is greater than the density of water.
While the density of the wood block is less than the density of the water.
An object will float if its density is less than the density of the fluid it is placed in.
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Reshape the steel block into the shape of a hull filled with air
Now the same mass has a larger volume
Since the volume has increased the density will decrease and air is less than the density of water
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Pressure:• Pressure = Force / Area• Earth’s atmosphere exerts pressure all
around us
According to Pascal’s principle, pressure applied to a fluid is transmitted throughout the fluid.
• Ex. Squeezing toothpaste
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Hydraulic machines that move heavy loads
The pressure applied to the small cylinder is transferred to the large cylinder
Since pressure remains constant throughout the fluid there is more force available to life the heavy load.
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Calculate Forces Page 487
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Bernoulli studied the properties of moving fluids such as water and air
Bernoulli’s principle: • As the velocity of a fluid increases, the
pressure exerted by the fluid decreases.
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Bernoulli’s Principle Example:
• Take a piece of paper and blow across the top of it
• The paper will rise
• This is because the velocity of the air over the paper is greater than the air below the paper
• The air pressure pushing down on the paper is lower than the pressure pushing upward
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Bernoulli’s Principle We use this principle today when
designing aircraft wings
We also use this principle when spraying fertilizers, herbicides, and insecticides to our yards.• A hose-end spray works by Bernoulli
principle
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Fluid Flow Fluids have the ability to flow
Viscosity:• The resistance to flow by a fluid
Fluids vary in their tendency to flow• Water has a low viscosity because it flows
easily• Cold syrup has a high viscosity because it
flows slowly
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Fluid Flow When we tip a container to begin flow
the flowing particles transfer energy to the stationary particles
Flowing particles pull the other particles
If the flowing particles do not effectively pull the other particles it has a high viscosity or high resistance to flow
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Fluid Flow If the flowing particles can easily pull
the other particles into motion then the liquid has a low viscosity or a low resistance to flow!