Chapter 11 Section 2: Floating and sinking• Key concepts: what is

the effect of the buoyant force? How can you use density to determine whether an object will float or sink in a fluid?

• Key terms: buoyant force, Archimedes’ principle, density

• http://www.youtube.com/watch?v=zvRJ9FKTdC8

Buoyancy• Buoyancy – ability to float • Buoyant force – upward force that

water and other fluids exert– Acts in a direction opposite to the

force of gravity, so it makes an object feel lighter

• Fluid exerts pressure on all surfaces of a submerged object

• Since pressure in a fluid increases with depth, the upward pressure on the bottom of the object is greater than the downward pressure on top. This is the buoyant force

Buoyant force

• Weight of a submerged object is a downward force

• If an object’s weight is greater than the buoyant force, a net force acts downward on the object and the object will sink.

• If weight is equal to the buoyant force, it floats.

Archimedes’ principle

• Archimedes – mathematician in ancient Greece• His principle states that the buoyant force

acting on a submerged object is equal to the weight of fluid the object displaces (this is why stuff floats – like ships – look at figure 10 page 383)

• A large object displaces more fluid than a small object.

Density• Why do some things float and others

sink?

• Density: mass per unit volume (d= m/v)

• By comparing densities, you can predict whether an object will float or sink in a fluid.

• The density of water is 1.00 g/cm3

• If something has a greater density, so greater than 1, it will sink in water (look at figure 11, pg 383). If it has a smaller density, like wood, it will float.

Changing Density• Changing density can

explain why an object floats or sinks. For example, ice is less dense than water. Why?

• When water freezes, the ice expands, so it occupies more space (takes up more volume).

• You can make an object sink or float in a fluid by changing its density.

This is everything you will need to know for the chapter quiz next week

• Your quiz will be Wednesday• Start reviewing – get a good grade – improve

your grade – smile all the way home – feel fantastic about yourself -