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Buoyancy

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**Buoyant Force pushes up on all matter when in a fluid.**

Buoyant force is equal to the weight of the volume of the fluid displaced by the object This is known as Archimedes’ principle Volume of rock = 1000 cm3 Volume of displaced water Buoyant force on rock = 9.8 N Weight of displaced water

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**The object’s volume determines the buoyant force**

The strength of the buoyant force on an object depends on the volume of the object that is underwater

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**So you could have a basketball full of air…. **

and a basketball of cement… …and the buoyant force would be the same if they were both completely submerged!!! 100 N 100 N

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**If the ball weighs more than the buoyant force, it will sink**

If the ball weighs more than the buoyant force, it will sink! If it weighs less, it will float! Weight Greater weight = sinking! Buoyant Force! Water Tank

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**This works with all fluids**

In air, hot air balloons create buoyant force that overcomes gravity’s pull downward. This is why astronauts train in swimming pools – the buoyant force simulates low-gravity!

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**Water Displacement and boats**

Boats float because they displace lots of water Increasing the volume without increasing the mass decreases the density. Hollow steel box Steel cube Volume = 10 cm3 Mass = 7.8 g Density = .78 g/cm3 Volume = 1 cm3 Mass = 7.8 g Density = 7.8 g/cm3

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**Sink or float? Water has a density of 1 gram/milliliter**

If an object is denser than this, it will sink, less dense and it will float.

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**Calculating density Density is equal to mass divided by volume.**

So a piece of wood with a mass of 200g and a volume of 400 ml has a density of 0.5 g/ml – And will float! V = m D D = m V m = DV

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**It really comes down to density**

Density is how much things are packed together. A cube filled with styrofoam is lighter than a cube filled with lead. This is because lead has a higher density than styrofoam. The unit for density is expressed using a mass unit divided by a volume unit, such as g/cm3, g/mL, kg/m3, kg/L (1 mL = 1 cm3) styrofoam lead

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**Density is a property of matter**

The density of an object such as steel is the same throughout. If you have a 1000 g block of steel with a density of 7.8 g/cm3 and cut it in half, each half will also have a density of 7.8 g/cm3. Mass = 1000 g Volume = 128 cm3 Density = 7.8 g/cm3 Mass = 500 g Volume = 64 cm3 Density = 7.8 g/cm3

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Try these…. 1. What is the density of a toy sub with a mass of 55g and a volume of 5 ml? 2. What is the mass of a lump of metal with a volume of 27 ml and a density of 8 g/ml? 3. How much volume is occupied by a rubber snake with a density of 3.4 g/ml and a mass of 380g? Will it float or sink in your neighbor’s swimming pool?

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