Buoyancy

Buoyancy Buoyancy is the ability to float.

Buoyant Force Buoyant force is when a fluid exerts an upward force.

Buoyant Force The buoyant force acts in the direction opposite to the force of gravity, so it makes an object feel lighter.

Archimedes’ Principle The buoyant force acting on a object is equal to the weight of the volume of fluid displaced by the object.

Archimedes’ Principle For example: If a boat displaces 10,000 lbs of water the buoyant force will be pushing up 10,000 lbs.

Displacement To displace means to move something with force. Like dominos knocking each other over… People who have to leave their homes because of a war… Or dogs displacing water when jumping into a pool…

Archimedes’ Principle For example: If a boat displaces 10 lbs of water the buoyant force will be pushing up will be 10 lbs.

Archimedes’ Principle So how does a 10,000 ton aircraft carrier float? Ship designers shape ships so that they displace a lot of water. The more water they displace, the greater the force of buoyancy.

Sink or Float If the buoyant force is greater than the weight of the object, it will float. Force of Buoyancy > Weight = Float

Sink or Float If the buoyant force is less than the weight of the object, it will sink. Force of Buoyancy < Weight = Sink

Sink or Float For Example: A boat weighs 250 lbs. The force of buoyancy is 300 lbs. Does it sink or float? Force of Buoyancy = 300 lbs Weight of object = 250 lbs. Force of Buoyancy > Weight of the object Floats!

Sink or Float For Example: A raft weighs 150 lbs. The force of buoyancy is 500 lbs. Does it sink or float? Force of Buoyancy = 500 lbs Weight of object = 150 lbs. Force of Buoyancy > Weight of the object Floats!

Sink or Float For Example: A box weighs 50 lbs. The force of buoyancy is 30 lbs. Does it sink or float? Force of Buoyancy = 30 lbs Weight of object = 50 lbs. Force of Buoyancy < Weight of the object Sinks!

Sink or Float For Example: A boat weighs 10 lbs. A kid weighing 50 lbs sits on the boat. The force of buoyancy is 150 lbs. Does it sink or float? Force of Buoyancy = 150 lbs Weight of boat = 10 lbs. Weight of kid = 50 lbs. Total Weight of object = 60 lbs. Force of Buoyancy > Weight of the object Floats!

Sink or Float Force of Buoyancy < Weight = Sink Force of Buoyancy > Weight = Float

Density Density tells you how much stuff is packed into an object. More dense objects are: Less dense objects are:

Density Density = Mass Volume Density of water = 1.0

Finding Density If a piece of wood has a mass of 28 g in a volume of 40 cm2, what is its density? Mass = 28 g volume = 40 cm2 Density = Mass Volume Density = 28 g = .7 g/cm2 40 cm2

Finding Density If a rock has a mass of 50 g in a volume of 4 cm2, what is its density? Mass = 50 g volume = 4 cm2 Density = Mass Volume Density = 50 g = 12.5 g/cm2 4 cm2

Density Comparing densities, you can predict whether an object will float or sink in a fluid. If object is more dense than fluid – it sinks If object is less dense than fluid - it floats An object with a density equal to that of the fluid floats at a constant depth.

Density