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 objectThis is known as

Archimedes’ principle

Volume of rock = 1000 cm3

Volume of displaced water

= 1000 cm3

Buoyant force on rock = 9.8 N

Weight of displaced water

= 9.8 N

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

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

If the ball weighs more than the buoyant

force, it will sink! If it weighs less, it will float!

Greater weight = sinking!

Weight

Water Tank

Buoyant Force!

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!

Water Displacement and boats

Boats float because they displace lots of water Increasing the volume without increasing the

mass decreases the density.

Volume = 10 cm3

Mass = 7.8 gDensity = .78 g/cm3

Volume = 1 cm3

Mass = 7.8 gDensity = 7.8 g/cm3

Steel cubeHollow steel box

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.

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!

D = mV

m = DV V = mD

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

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 gVolume = 128 cm3

Density = 7.8 g/cm3

Mass = 500 gVolume = 64 cm3

Density = 7.8 g/cm3

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?