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Chapter 14 Buoyancy Slide 2 Slide 3 Buoyancy in a Liquid buoyant force or buoyancyThe downward weight of an object (force due to gravity) is not as great as an upward force on that object called the buoyant force or buoyancy Weight Slide 4 Buoyancy in a Liquid The force of pressure is greater at the bottom of the object than the force of pressure exerted on the top of the object. Slide 5 Therefore, the buoyant force is due to the pressure differences between the top and the bottom of object. Slide 6 Buoyancy arises from the fact that: fluid pressure increases with depth the increased pressure is exerted in all directions there is an unbalanced upward force on the bottom of a submerged object Slide 7 Buoyant Force Equals Weight of Liquid Displaced any object placed in water displaces a certain amount of water think about how the water level in the bathtub rises when you get in you can use the weight of the displaced water to determine the buoyant force Slide 8 Buoyant Force Equals Weight of Liquid Displaced What is the buoyant force on the weight? To calculate, figure out the weight of the displaced water. Slide 9 Slide 10 Archimedes Principle Archimedes lived over 2000 years ago in Greece. He discovered the fact that: The buoyant force on an object is equal to the weight of the fluid displaced by the object. Slide 11 Sink or Float? The buoyant force determines whether an object will sink or float. If the buoyant force is greater than the weight of the object, then the object will float If the buoyant force is less than the weight of the object, then the object will sink. Slide 12 Sink or Float? An object floats when it displaces a volume of fluid whose weight is greater than or equal to its own weight. An object will float in a fluid if the density of that object is less than the density of the fluid. Slide 13 Why is the tip of the iceberg the only part seen out of the ocean? Slide 14 Isostasy The buoyant force of the ocean pushes the iceberg upwards, but the volume of the ice is only slightly less than the same volume of salt water it displaces, so almost 90% of the iceberg remains submerged. Slide 15 Why do some objects sink while others float? Density (the objects mass divided by its volume - how much space it takes up) In order for an object to float, the water it displaces must weigh more than the object itself Or to put in density terms, the object must have a density lower than the density of the water Slide 16 If the density of water is 1g/cm 3, then... will wood (D = 0.8 g/cm 3 ) float? will aluminum (D = 2.7 g/cm 3 ) float? will steel (D = 7.8 g/cm 3 ) float? Slide 17 Didnt you just say steel would sink?!? Then why is this ship made of steel floating on top of the water? Slide 18 The shell of the ship may be made of steel, but most of the space inside the hull is filled with air that has a very low density. Slide 19 Some creatures in the sea have gas filled bladders whose volume can be changed to adjust for the buoyant force at various depths. Slide 20 This is how submarines work too. They can take in sea water to submerge or discharge sea water to rise up to the surface. Slide 21 Can you think of something else that uses changes in the density within a chamber to make it rise or fall in our atmosphere?