1. buoyant force and fluid pressure

2. Imagine that you have a glass of ice water in front of you
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?

3. 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.

4. 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)

5. 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.

6. Zach
I’m so confused!!
Any student’s name with a good sense of humor :P

7. 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.

8. 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

9. 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.

10. 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

11. 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.

12. So, will an object sink or float?
That depends on whether the buoyant force is less than or equal to the object’s weight.
Kids may think back to what was learned about density….that information will come next. How density is related to floating objects.

13. 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.
The buoyant force is 15N because according to Archimedes’s the buoyant force equals the weight of the volume of fluid that the object displaces. The object will sink because its weight is more than the buoyant force

14. Videos http://www.youtube.com/watch?v=2afDLk-JzEY&feature=related
Bill Nye the Science Guy video
Worksheet available with Bill Nye video