2 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’ principleVolume of rock= 1000 cm3Volume of displaced waterBuoyant force on rock= 9.8 NWeight of displaced water
3 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
4 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 N100 N
5 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!WeightGreaterweight =sinking!BuoyantForce!Water Tank
6 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!
7 Water Displacement and boats Boats float because they displace lots of waterIncreasing the volume without increasing the mass decreases the density.Hollow steel boxSteel cubeVolume = 10 cm3Mass = 7.8 gDensity = .78 g/cm3Volume = 1 cm3Mass = 7.8 gDensity = 7.8 g/cm3
8 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.
9 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 =mDD =mVm =DV
10 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)styrofoamlead
11 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 cm3Density = 7.8 g/cm3Mass = 500 gVolume = 64 cm3Density = 7.8 g/cm3
12 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?