Cornell Notes 4.2 Buoyancy January 18, 2012 Pages 125 and 127

Weight and buoyancy Buoyancy is a measure of the upward force a fluid exerts on an object that is submerged. The water in the pool exerts an upward force that acts in a direction opposite to the boy’s weight.

Weight and buoyancy The strength of the buoyant force on an object in water depends on the volume of the object that is underwater. As you keep pushing downward on the ball, the buoyant force gets stronger and stronger. Which ball has more volume underwater?

Weight and buoyancy 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

Weight and buoyancy 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.

Big Ol’ Warning 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

Weight and buoyancy These blocks are the same total volume. Which block has more buoyant force acting on it? Which block weighs more in air?

Weight and buoyancy 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.

Buoyancy 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 vs Buoyancy Weight > Buoyancy SINKS Negatively Buoyant

Weight = Buoyancy STAYS Neutrally Buoyant

Density and buoyancy If you know an object’s density you can quickly predict whether it will sink or float. Which ball will sink in water? Which ball will float in water?

Density and buoyancy 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. What can you say about the average density of these blocks?

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

Boats and average density 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.