Why do things sink or float

1
Why do things sink or float?

• Consider that a small key will fall to the bottom
  of a pool of water but a huge ship made of steel
  will float
• The key only weighs a few ounces and sinks but a
  ship weights thousands of pounds and floats
• How come?

2
What is density?

• Thin about that a moment. What does density mean?
  A beach ball takes up more space than a bowling
  ball and yet.
• Which is more dense?
• WHY?

3
Which weighs more??

• Iron or feathers?
• Styrofoam or steel?

4
Density block lab for C-1

• Another practice for measuring and drawing
  conclusions follows
• Density Block Lab
• Choose a team of 2 or 3 and as block sets become
  available carry out this lab in 3 steps.

5
BUTwith certainty you can be sure

• Which is more dense, iron or feathers?
• Which is more dense styrofoam or steel?

6
Weight vs Density !

• Are they the same?
• How are they different?
• How are they related?

7
Lets consider water

• As our standard, water has a density of 1 g/
  cubic centimeter
• A few ounces of a key will sink because its
  density is more than 1 g/cc
• A few thousand metric tons of ship will float
  because its density is LESS than 1 g/cc

8
Density is

• TWO factors acting togethernot one or the
  otherbut TWO
• Density depends on how much matter you have in a
  certain amount of space.
• If that ration of matter to space is a number
  greater than 1 gram of mass packed into one cubic
  centimeter of space then the object will sink in
  water!
• If however, less than one gram of mass occupies 1
  cc of space then it will float on water!!

9
And so

• The key sinks because there is MORE than a gram
  of atoms packed into 1 cc of space.
• A ship floats because if you consider the whole
  volume of a huge shipthe space it takes upand
  divide that into the mass of the whole ship the
  number will be LESS than 1 gram of atoms packed
  into1 cc of space.

10
Density and temperature

• Density is NOT dependent on the amount of the
  sample of a substance.
• Density DOES depend on temperature. So when we
  compare the densities of different things we must
  discuss them all at 1 atm of pressure at some
  constant temperature.
• When we heat something, the temperature
  increases. It increases because the molecules of
  the substance start to move faster and faster and
  they move apart from one another so that the mass
  doesnt change, the number of atoms hasnt
  changed they have just moved father apart, so
  obviously the volume does change. It increases
  since the atoms are moving farther apart.
• D mass/volume so density decreases.

11
Lab step one

• Using construction paper glue or tape and
  scissors I want you to build the following cubes
• Build a 1 cubic centimeter cube
• Build a 2 cubic centimeter cube
• Build a 4 cubic centimeter cube.
• Make three statements of observation and tape
  your cubes on a piece of construction paper for
  presentation.
• 15 points.
• Due____________________

12
Lab step 2

• I will give you several blocks of unidentified
  material. You will measure the density of each
  block AND indicate whether it will float or sink
  in water. If the block were twice the size
  indicate whether it would still sink or float.
  Show the math to prove your answer.
• 10 points
• Due__________________

13
Lab step 3

• I am going to give you a list of densities of
  various materials. Using tape I want you to
  identify each as letters A through I.
• 1.1 -1.2 g/cc
• 2.7 g/cc
• 8.5 g/cc
• 8.9 g/cc
• 0.60 0.90 g/cc
• 0.45 g/cc
• 0.90 0.92 g/cc
• 1.4 g/cc
• 7.9 g/cc
• 18 points
• Due_____________________