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# Buoyancy

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### Buoyancy

1. 1. Ups and Downs: Buoyancy Oh buoy this is going to be fun
2. 2. The Ups and Downs <ul><li>What are some examples of buoyant objects? </li></ul>
3. 3. <ul><li>If one places a copper ball in a pail of water it will sink, whereas a wooden ball will float. Whether or not a given object will sink or float in a fluid is determined by the buoyant force on the object. </li></ul>
4. 4. Forces Acting on Buoyancy <ul><li>The buoyant force is caused by the difference between the pressure at the top of the object ( gravitational force ), which pushes it downward, and the pressure at the bottom ( buoyant force ), which pushes it upward </li></ul><ul><li>Since the pressure at the bottom is always greater than at the top, every object submerged in a fluid feels an upward buoyant force. </li></ul>
5. 5. Buoyant force Buoyant force on the rock is small Gravitational force on the rock is big (as the density of the rock is more than that of the water) Displaced water Buoyant force on the toy boat is big! ( total Density is less than that of the water) Gravitational force on the toy boat is small
6. 6. Gravitational Force <ul><li>Objects also feel a downward force due to gravity </li></ul><ul><li>The difference between the gravitational force and buoyant force on a object determines if the object will sink, or float </li></ul><ul><li>If the weight is greater than the buoyant force, the object sinks, and vice versa. </li></ul>
7. 7. <ul><li>so that the difference between the buoyant force and the weight is determined by the relative density of the object and the fluid. In particular, we come to Archimide's principle , which implies that </li></ul>
8. 8. Archimedes Principle <ul><li>This was discovered by Archimedes in approx 250 BC and it is now called the Archimedes Principle. </li></ul><ul><li>Story goes that the king thought that his gold smith was stealing the gold from his crown and hired Archimedes to prove it. </li></ul><ul><li>Archimedes was in the bath one night and he had a revelation… Thus, he reasoned that the buoyant force on an object must be equal to the weight of fluid that object displaces </li></ul>
9. 9. <ul><li>If the weight of an object is greater than the weight of displaced fluid, it will sink, wherease if the weight of the object is less than the weight of displaced fluid, it will rise (density). </li></ul><ul><li>if a bar of gold will not float and it should have the same density as the king ’ s crown (just a different shape) than the same amount of water should be displaced </li></ul>
10. 10. This is what happened… <ul><li>Archimedes put the crown and the gold into the bath tub and look what happened. Which one has more density? </li></ul>
11. 11. <ul><li>so that the difference between the buoyant force and the weight is determined by the density of the object </li></ul><ul><li>It ’s cumulative density was less than the gold bar…the crown was not pure gold </li></ul>
12. 12. <ul><li>Word has it that Archimedes was so excited about his discovery that he jumped out of the bath and ran down the street naked yelling “ Eureka ” (or “ I have found it ” ) </li></ul><ul><li>His discovery is still used today and it is called the Archimedes Principle – </li></ul><ul><li>The buoyant force on an object immersed in a fluid is equal to the weight of a fluid the object displaces </li></ul><ul><li>Or </li></ul><ul><li>A solid object will sink in a fluid if it ’ s density is greater than the fluids density and will float if it is less than the fluid ’ s density </li></ul>
13. 13. Which one would have more buoyant force?