Density

What is density and how is it measured? Density

1. Density a.describes how much mass is in a given volume of a material.

b. Solids, liquids and gases are matter, so they all have density. c. The units used for density depends on whether the substance is solid or liquid. –For liquids use units of grams per milliliter (g/mL) –For solids use density in units of g/cm 3 g/cm 3

The density of a hollow plastic toy is less than water, and lower density materials will float above higher density materials.

d. Density is a property of material that is independent of quantity or shape. Steel Nail

e. density changes for different substances because: i. Atoms have different masses. ii. Atoms may be “packed” tightly or loosely.

f. Liquids tend to be less dense than solids of the same material. i. Water is an exception to this rule. The density of solid water, or ice, is less than the density of liquid water.

Think: Why might this be important for lake life where lakes freeze in the winter? allows creatures to survive the cold temperatures

2. Volume a. the amount of space an object takes up. b. The volume of a rectangular solid is found by multiplying length times width times height. length width height

c. Use a graduated cylinder to measure the volume of a liquid.

d. To measure the volume of an irregular shape, use a technique called displacement. i. put the object in water and measure the amount of water displaced (how much did the volume change?)

3. Finding Volumes of Common Shapes

Steel is more dense than water so why do steel boats float? Buoyancy

1. Weight a. Weight is a force, like any other pushing or pulling force, and is caused by Earth’s gravity. b. it is easy to confuse mass and weight, but they are not the same. c. Weight is the downward force of gravity acting on mass.

2. Buoyancy a. Buoyancy is a measure of the upward force a fluid exerts on an object that is submerged. The water in the pool exerts an upward force that acts in a direction opposite to the boy’s weight.

b. The strength of the buoyant force on an object in water depends on the volume of the object that is underwater. As you keep pushing down on the ball, the buoyant force gets stronger and stronger. More volume underwater Less volume underwater

Diving Example … adjust buoyancy by adjusting air in BC (buoyancy compensator).

c. In the third century BC, a Greek mathematician named Archimedes realized that buoyant force is equal to the weight of fluid displaced by an object. d. A simple experiment can be done to measure the buoyant force on a rock with a spring scale.

e. Buoyancy explains why some objects sink and others float. Buoyant force is stronger than weight  Object floats Weight is stronger than buoyant force  Object sinks

3. Practice Problem – Buoyancy a. What is the buoyant force on a rock with a volume of 1,000 cm 3 ? 1. In air, the scale shows the rock’s weight as 29.4 newtons. 2. When the rock is completely submerged, the scale reads 19.6 newtons. 3. The difference is a force of 9.8 newtons. This is the buoyant force!

4. Density and buoyancy a. If you know an object’s density you can quickly predict whether it will sink or float. Which ball will sink in water? Which ball will float in water?

b. Average density determines whether objects sink or float. –An object with an average density GREATER than the density of the liquid will sink. –An object with an average density LESS than the density of the liquid will float. What can you say about the average density of these blocks?

c. steel boats can be made to float because their density is less than water

d. a loaded cargo ship will sit lower in the water than an unloaded ship nearby. - A full ship has more mass than an empty ship. - A full ship must displace more water (sink deeper) to make the buoyant force large enough to balance the ship’s weight.

These blocks are the same total volume. Which block has more buoyant force acting on it? Which block weighs more in air?