Cornell Notes 4.2 - Buoyancy

Cornell Notes 4.2 - Buoyancy

January 18, 2012 - Pages 125 and 127

•  Buoyancy is a measure of the upward force a fluid exerts on an object that is submerged.

•  The strength of the buoyant force on an object in water depends on the volume of the object that is underwater.

•  In the third century BC, a Greek mathematician named Archimedes realized that buoyant force is equal to the weight of fluid displaced by an object.

•  This is called Archimedes’ Principle

•  What is the buoyant force on a rock with a volume of 1,000 cm3?

•  In air, the scale shows the rock’s weight as 29.4 newtons.

•  When the rock is completely submerged, the scale reads 19.6 newtons.

•  The difference is a force of 9.8 newtons.

•  There is a BIG difference between the weight of and object and the weight of the water it displaces

•  If an object has a volume of 1000 cm3 and a mass of 5000 g (which weighs 49.0 N), it doesn’t mean that the water pushes up with 49 N but only 9.8, which is the weight of 1000 cm3 and therefore 1000 g (9.8 N) of water

•  Buoyancy explains why some objects sink and others float.

•  Whether an object sinks or floats depends on how the buoyant force compares with the weight.

•  If something sinks, we say that it is negatively buoyant

•  If something floats, we say that it is positively buoyant

•  If something stays in the middle of the water, we say that it is neutrally buoyant

•  Weight < Buoyancy FLOATS Positively Buoyant

•  Weight > Buoyancy SINKS Negatively Buoyant

•  Weight = Buoyancy STAYS Neutrally Buoyant

•  If you know an object’s density you can quickly predict whether it will sink or float.

•  Average density determines whether objects sink or float.

–  An object with an average density GREATER than the density of water will sink.

–  An object with an average density LESS than the density of water will float.

•  Use your understanding of average density to explain how steel boats can be made to float.

•  If you have seen a loaded cargo ship, you might have noticed that it sat lower in the water than an unloaded ship nearby.

•  A full ship has more mass than an empty ship.

•  This means a full ship must displace more water (sink deeper) to make the buoyant force large enough to balance the ship’s weight.