Chapter 7 Forces in Fluids Section 2 Buoyant Force

Essential Questions 7-2 What is the relationship between fluid pressure and buoyant force? How can you predict whether an object will sink or float in a liquid? What role does density play in an object’s ability to float? How can the overall density of an object be changed?

Buoyancy Net upward force is called the buoyant force Easier to lift a rock in water

Displacement of Water The amount of water displaced is equal to the volume of the rock.

Archimedes’ Principle An immersed body is buoyed up by a force equal to the weight of the fluid it displaces. The apparent weight of an object in a liquid is gravitational force (weight) minus the buoyant force

Sinking If the buoyant force < the force of gravity, then the object will sink

Flotation A floating object displaces a weight of fluid equal to its own weight.

Flotation If the buoyant force = the force of gravity, then the object will float

Discussion Why can a duck float on water but not on air?

Buoying Up If the buoyant force > the force of gravity, then the object will be buoyed up (pushed to the surface)

Buoyancy in a Gas An object surrounded by air is buoyed up by a force equal to the weight of the air displaced. Exactly the same concept as buoyancy in water. Just substitute air for water in the statement If the buoyant force is greater than the weight of the object, it will rise in the air

Buoyancy in a Gas Since air gets less dense with altitude, the buoyant force decreases with altitude. So helium balloons don’t rise forever!!!

Discussion You are part of a team that is raising a sunken ship from deep underwater. Several members of the team suggest inflating large balloons inside its hull. Do you agree with them? Why or why not?

Changing Overall Density A ship floats because of its’ shape Submarines have ballast tanks to fill with water, this allows the sub to increase its’ density to sink under the surface Fish use a swim bladder to change their density so they can go up and down in the water

7-2 Mini Quiz: 5 Questions This pushes an ice cube to the top of a glass of water a. a sinking object b. a floating object c. a buoyed-up object d. buoyant force e. density

This has a weight of 10 N and a buoyant force of 10 N a. a sinking object b. a floating object c. a buoyed-up object d. buoyant force e. density

This has a weight of 15 N and a buoyant force of 18 N a. a sinking object b. a floating object c. a buoyed-up object d. buoyant force e. density

This can be decreased by making the shape hollow a. a sinking object b. a floating object c. a buoyed-up object d. buoyant force e. density

This has a density of 2.5 g/cm3 in water a. a sinking object b. a floating object c. a buoyed-up object d. buoyant force e. density