Buoyancy and DensityBuoyancy and Density

14-214-2

Buoyant ForceBuoyant Force

• Buoyant force = upward force that keeps an object immersed in or floating on a liquid

• It’s the force that pushes an ice cube up to the surface of water

• Buoyant force = upward force that keeps an object immersed in or floating on a liquid

• It’s the force that pushes an ice cube up to the surface of water

Buoyant Force and Fluid PressureBuoyant Force and Fluid Pressure

• Water exerts pressure all around an object

• Horizontally the pressures are equal and cancel each other out.

• Vertically, the deeper the object, the more pressure exerted

• The force pushing upward is the buoyant force

• Water exerts pressure all around an object

• Horizontally the pressures are equal and cancel each other out.

• Vertically, the deeper the object, the more pressure exerted

• The force pushing upward is the buoyant force

Determining Buoyant ForceDetermining Buoyant Force

• Archimedes was a mathematician who lived in 3rd century BCE and discovered a way to determine buoyant force.

• Archimedes's Principle = buoyant force is equal to the weight of the volume of fluid the object displaces

• Archimedes was a mathematician who lived in 3rd century BCE and discovered a way to determine buoyant force.

• Archimedes's Principle = buoyant force is equal to the weight of the volume of fluid the object displaces

Cont….Cont….• So…

– 1st determine amount of liquid displaced

– 2nd determine how much that liquid weighs

– 3rd weight = buoyant force

• So…– 1st determine

amount of liquid displaced

– 2nd determine how much that liquid weighs

– 3rd weight = buoyant force

Weight vs Buoyant ForceWeight vs Buoyant Force

• Object will sink if its weight is greater than the buoyant force acting on it

• Object will float if weight is equal to buoyant force

• Buoying Up = object under water is pushed up until the part under is equal to the amount it displaces

• Object will sink if its weight is greater than the buoyant force acting on it

• Object will float if weight is equal to buoyant force

• Buoying Up = object under water is pushed up until the part under is equal to the amount it displaces

Density and FloatingDensity and Floating• Object is more

dense than its surroundings = sink

• Object is less dense than surroundings = float

• Object is more dense than its surroundings = sink

• Object is less dense than surroundings = float

Math Focus! Page 415Math Focus! Page 415

• Complete 1, 2,3 on your paper1. D = 25g ÷ 20cm3

1. D = 1.25 g/cm3

2. D = 546g ÷ 420cm3

1. D = 1.3 g/cm3

• D = 163g ÷ 50cm3

1. D = 3.26 g/cm3

• Complete 1, 2,3 on your paper1. D = 25g ÷ 20cm3

1. D = 1.25 g/cm3

2. D = 546g ÷ 420cm3

1. D = 1.3 g/cm3

• D = 163g ÷ 50cm3

1. D = 3.26 g/cm3

Determining DensityDetermining Density

• Need mass and volume!• Volume of a regular solid = L*W*H

– (length*width*height)

• Volume of irregular solid = use displacement.– Measure volume of liquid , add object,

measure new volume, subtract to get volume of object.

• Need mass and volume!• Volume of a regular solid = L*W*H

– (length*width*height)

• Volume of irregular solid = use displacement.– Measure volume of liquid , add object,

measure new volume, subtract to get volume of object.

Changing Overall DensityChanging Overall Density

1. Changing shape can change density- see page 417

2. Changing mass (ex.submarine)3. Changing volume (ex. Swim

bladder)

1. Changing shape can change density- see page 417

2. Changing mass (ex.submarine)3. Changing volume (ex. Swim

bladder)