436 Views

Download Presentation
## Buoyant force

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -

**Buoyant force**buoyant force and fluid pressure**Imagine that you have a glass of ice water in front of you.**Take your straw and push down the ice cubes. • What happens to the ice cubes?**A force pushes the ice cubes back up**• This force is called buoyant force • Buoyant force- the upward force that keeps an object immersed in or floating on a liquid.**Buoyant force and fluid pressure**• Water exerts pressure on all sides • The pressure on the sides is equal- they cancel • Water pressure pushes down and buoyant force pushes up. The object goes in the direction of the greatest force (N)**Determining Buoyant force**• Archimedes, a Greek mathematician discovered how to determine buoyant force. The buoyant force on an object in a fluid is an upward force equal to the weight of the fluid that the object takes the place of, or displaces.**Zach**I’m so confused!!**The buoyant force on an object in a fluid is an upward force**…….. equal to the weight (N) of the fluid that the object takes the place of, or displaces.**So how do you find out buoyant force?**• Determine the displacement (volume of the object) • Then, take the weight of that volume of fluid and change it to Newtons • 1 kg = about 10 N (9.8 to be more exact) • The buoyant force = the weight in newtons of the displaced fluid**Sinking**• An object will sink if its weight is greater than the buoyant force. • For example: • A rock weighs 75N and it displaces 5L of water. So, the weight of the displaced water is 50N. • The rock’s weight is great than the buoyant force, so the rock sinks.**Floating**• An object floats only when the buoyant force on the object is equal to the object’s weight. • For example: • The fish weights 12N and displaces a volume of water that weighs 12N. • Because they are equal, the fish can float**Buoying up**• When the buoyant force on an object is greater than the object’s weight, the object is buoyed up (pushed up) in water • For example: • If a duck dives into water it displaces more than 9N of water, which is the buoyant force. So, the buoyant force on the duck would be greater than the duck’s weight • An object is buoyed up until the part of the object under water displaces an amount of water that equals the object’s entire weight.**So, will an object sink or float?**• That depends on whether the buoyant force is less than or equal to the object’s weight.**Review**• An object weights 20N. It displaces a volume of water that weights 15N. • What is the buoyant force on the object? • Will this object float or sink? Explain.**Videos**• http://www.youtube.com/watch?v=2afDLk-JzEY&feature=related • http://www.youtube.com/watch?v=y0SnFCs9z1g&feature=related • Bill Nye the Science Guy video • http://www.youtube.com/watch?v=-U1YVZMifMM&feature=related