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