Density and Buoyancy 10 27 08 Density The

Density and Buoyancy 10 -27 -08

Density • The amount of mass or matter in a given volume • Characteristic property – identifies the substance • Formula – D = m/v • Units – g/cm 3, or kg/m 3, or g/ml for a liquid

A little side note on volume • Volume is the amount of space an object takes up • Solid objects – cm 3 • Liquids - ml • Regular shaped objects (cubes, rectangular prism) – l x w x h • Irregular shaped objects – water displacement method Final volume – initial volume

Questions • 1. What is the density of a cube that has a • • volume of 6 cm 3 and has a mass of 18 grams? 2. What is the density of a 10 g irregular shaped block, that when you put it in a beaker filled with 10 ml of water, rises to 12 ml? 3. Find the density of a liquid that has a mass of 24 grams and a volume of 6 ml.

Density • A ratio – does not change (Mass cannot change without volume changing also, proportionately) • Density of water 1 g/ml or 1 g/cm 3

Question • What has a larger density – 1 L of water or 10 L of water?

Sinking and Floating • When an object is • more dense than the liquid you put it in, it SINKS When an object is less dense than the liquid you put it in, it FLOATS

Density Can Change with…… • Temperature! • Molecules move around and expand with heat, increasing volume

Density Applications • • • Plate tectonics Continental Crust: 2. 7 to 3. 0 Oceanic Crust: 3. 0 to 3. 3 Mantle (silicates): 3. 3 to 5. 7 (increasing with depth? ) Outer Core (liquid): 9. 9 to 12. 2 Inner Core (solid): 12. 6 to 13. 0 http: //www. youtube. com/watc h? v=ryr. XAGY 1 dm. E&feature=re lated

More applications • Density causes convection currents in the ocean • http: //www. youtube. com/watch? v=Fu. OX 2 3 y. Xh. Z 8&feature=related • http: //www. youtube. com/watch? v=n. AOSF AGOu. S 8&feature=related

Submarines • Submarines can sink or float • http: //express. howstuffworks. com/express -submarine 1. htm

Buoyancy • Buoyant Force - The upward force that fluids exert on all matter, caused by differences in fluid pressure

Describe the forces in the following picture • Which way does the buoyant force act upon the block?

Archimedes Principle • Archimedes Principle - the buoyant force on a submerged object is equal to the weight of the fluid that is displaced by the object.

What is the buoyant force? • If an object when placed in water displaces 1 kg of water…. . what is the buoyant force on the object? • Hint: Buoyant force is equal to the weight (not mass) of the water displaced

Sinking and floating • Things float when the weight of water displaced (buoyant force) is equal to or more than the objects weight (gravity) • Things sink when the weight of the water displaced (buoyant force) is less than the weight of the object (gravity)

Changing shape changes volume • Changing the shape of an object can allow for more water displacement and changes the volume of an object • Object at left has density greater than water's; it sinks. • Object at the right has a density less than water; it floats. The large volume--most of it air-makes the density small (average density), less than that of water, so it floats