1. Dr. Jie Zou PHY 1151G Department of Physics1 Chapter 15 Fluids (Continued)

2. Dr. Jie Zou PHY 1151G Department of Physics2 Outline Buoyancy and origin of buoyant force Archimedes’ principle Applications of Archimedes’ principle Flotation: What makes an object sink or float?

3. Dr. Jie Zou PHY 1151G Department of Physics3 Buoyancy Buoyant force: A fluid exerts a net upward force on any object it surrounds. This is the buoyant force. Origin of the buoyant force: Pressure increases with depth.

4. Dr. Jie Zou PHY 1151G Department of Physics4 Archimedes's principle Archimedes's principle: An object completely immersed in a fluid experiences an upward buoyant force equal in magnitude to the weight of fluid displaced by the object. More generally, if a volume V of an object is immersed in a fluid of density , the buoyant force is: F b =  gV. The volume V may be the total volume of the object, or any fraction of the total volume.

5. Dr. Jie Zou PHY 1151G Department of Physics5 Application of Archimedes's principle: Example A person weighs 720.0 N in air is lowered into a tank of water to about chin level. He sits in a harness of negligible mass suspended from from a scale that reads his apparent weight. He now exhales as much air as possible and dunks his head underwater, submerging his entire body. If his apparent weight while submerged is 34.3 N, find (a) his volume and (b) his density.

6. Dr. Jie Zou PHY 1151G Department of Physics6 What makes an object sink or float? Floatation: Whether an object will sink or float in a liquid depends on how the buoyant force compares with the object’s weight. When an object floats, the buoyant force acting on it equals its weight. Consider these three simple rules: 1. If an object is denser than the fluid in which it is immersed, it will sink. 2. If an object is less dense than the fluid in which it is immersed, it will float. If an object has a density equal to the density of the fluid in which it is immersed, it will neither sink nor float.

7. Dr. Jie Zou PHY 1151G Department of Physics7 Homework See online assignment on www.masteringphysics.com www.masteringphysics.com