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Buoyancy

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What is

buoyancy?

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When we get into the water…

our weight exerts force on the water,

and the water pushes back on us.

This upward force is called buoyancybuoyancy

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Archimedes’ Principle

The buoyant force on an object in a fluid

is equal to the weight of the fluid that the object displaces.

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Archimedes’ Principle

N

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TO DETERMINE BUOYANT FORCE

BUOYANT FORCEBUOYANT FORCE =

WEIGHT IN AIR (N) - WEIGHT IN FLUID (N)

Example: If 300 g of modeling clay weighs 2.9 N in air and 1.6 N in water, the buoyant force acting on it is…….?

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BuoyancyBuoyancy

1)1) An object suspended in a liquid An object suspended in a liquid experiences an upward force and water experiences an upward force and water is displaced.is displaced.

2) 2) Gravity (weight) and buoyancy are Gravity (weight) and buoyancy are forces that act in the opposite direction.forces that act in the opposite direction.

3) 3) Buoyancy is measured in NEWTONS!Buoyancy is measured in NEWTONS!

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When will it float?When will it sink?

There are two ideas that make a major difference!

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An object will sink or float

depending on whether it

weighs more or less than

the fluid it displaces.

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In other words…if the object is denser than the fluid around it,

the object will also weigh more than the fluid it displaced.

So, it will sink.

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If the object is less dense than the fluid around it,

the object will weigh less than the fluid it displaces.

So, it will float.

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Negative Buoyancy

• Gravity is greater than the buoyant force

• Object sinks!

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Positive Buoyancy

• Buoyant force is greater than gravity

• Object floats!

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Neutral Buoyancy

• Buoyant force and gravity are equal

• Object stays at same depth in fluid

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Buoyancy in Air

• Remember that air is a fluid therefore the same principles apply as in liquids.

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So, Why Do HEAVY Things Float?

We have to consider more than one variable:

MassDensityDesignShape

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Clay Boat ActivityClay Boat Activity

Answer the following questions:

1)How did the volume displaced by the ball of modeling clay differ from when it was a floating boat shape?

2) Why did the boat float and the clay ball sink?

3) If your modeling clay has a mass of 20 grams, what minimum mass of water must it have displaced to float?

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3) Adding pennies to you boat added mass and therefore weight or gravitational pull. If your boats mass is 20 grams and it displaced 45 milliliters of water, how many pennies could it theoretically hold before sinking?

4) Why did your boat sink?

5) How can you modify a dense solid substance to make it float in a less dense fluid?

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Remember:

•Buoyancy is a force opposite gravity and is measured in Netwons.

•Buoyancy relates to the amount of fluid the object displaces. You must take into account more than one variable: mass, density, shape, design.

•You can increase the volume of water an object will displace without changing the object’s mass.

Clay Boat & Clay Boat & Buoyancy ReviewBuoyancy Review

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Temperature, Viscosity, Temperature, Viscosity, Density & BuoyancyDensity & Buoyancy

Make a chart like the one below. Fill it in with arrows going up or down. If you think it goes up, point your arrow up, and if you think it goes down point your arrow down. When you finish, write and explanation.

VOLUME DENSITY VISCOSITY BUOYANCY

TEMPERATURE TEMPERATURE

TEMPERATURE TEMPERATURE

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Viscosity, density and buoyancy all change with temperature. As heat is taken away from a fluid, its particles slow down (lose energy) and come closer together. They have less energy to overcome the force of attraction which makes it more difficult for the particles to flow past each other. Therefore the viscosity increases.

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As the particles come closer together, this causes the fluid to contract and the volume decreases

REMEMBER: D = m / VREMEMBER: D = m / V

If the volume decreases and the mass stays the same, the density will increase as well. Therefore, the solid form of a substance is more dense than the liquid form.

BUT THERE IS AN EXCEPTION….BUT THERE IS AN EXCEPTION….

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THE EXCEPTION TO THE RULE…

Water behaves differently from Water behaves differently from other fluids. Ice is less dense than other fluids. Ice is less dense than liquid water. Water is most dense at liquid water. Water is most dense at 4 degrees C. This allows aquatic life 4 degrees C. This allows aquatic life to survive in the winter!to survive in the winter!

Sinking of Titanic

The boat that was unsinkable!

•Sinking of Titanic Part 1

•Sinking of Titanic Part 2

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