1. Buoyancy Force

2. Floating is a type of equilibrium An upward force counteracts the force of gravity for these objects. This upward force is called the buoyant force. F buoy mg

3. The Buoyant Force Archimedes’ Principle: a body immersed in a fluid is buoyed up by a force that is equal to the weight of the fluid displaced. F buoy =  Vg – F buoy : the buoyant force exerted on a submerged or partially submerged object. – V: the volume of displaced liquid. –  : the density of the displaced liquid. When an object floats, the upward buoyant force equals the downward pull of gravity. The buoyant force can float very heavy objects, and acts upon objects in the water whether they are floating, submerged, or even sitting on the bottom.

4. Buoyant force on submerged object mg F buoy =  Vg A sharks body is not neutrally buoyant, and so a shark must swim continuously or he will sink deeper.

5. Buoyant force on submerged object mg  Vg SCUBA divers use a buoyancy control system to maintain neutral buoyancy (equilibrium!).

6. Buoyant force on submerged object mg  Vg If the diver wants to rise, he inflates his vest, which increases the amount of water he displaces, and he accelerates upward.

7. Buoyant force on floating object mg  Vg If the object floats on the surface, we know for a fact F buoy = mg! The volume of displaced water equals the volume of the submerged portion of the ship.

8. Sample problem Assume a wooden raft has 80.0% of the density of water. The dimensions of the raft are 6.0 meters long by 3.0 meters wide by 0.10 meter tall. How much of the raft rises above the level of the water when it floats?

9. Sample problem You want to transport a man and a horse across a still lake on a wooden raft. The mass of the horse is 700 kg, and the mass of the man is 75.0 kg. What must be the minimum volume of the raft, assuming that the density of the wood is 80% of the density of the water.

10. Parking in St. Bernard Parish after Hurricane Katrina

13. “Mobile” Homes in St. Bernard Parish after Hurricane Katrina

15. Estimation problem Estimate the mass of this house in kg.