Chapter 2 – Section 2 Suggested Reading Pages 33-42 Units of Measurement.
-
Upload
kerry-scott -
Category
Documents
-
view
231 -
download
1
Transcript of Chapter 2 – Section 2 Suggested Reading Pages 33-42 Units of Measurement.
Chapter 2 – Section 2
Suggested Reading
Pages 33-42
Units of Measurement
Making Measurements
Measurements must have a number and a unit.
The unit depends on the quantity being measured.
SI Measurement
Le Systeme International d’Unites
See Table of SI Base units – Page 34
See Table of SI Prefixes – Page 35
Standard SI Units
Length = meter (m)
Mass = kilogram (kg)
Time = second (s)
Temperature = Kelvin (K)
Amount of substance = mole (mol)
Derived SI Units
Area = square meter (m2)
Volume = cubic meter (m3)
Density = kilograms per cubic meter (kg/m3)
Produced by multiplying or dividing standard units.
Definitions
Mass – a measure of the quantity of matter in an object.
Weight – a measure of the gravitational pull on matter.
Volume – the amount of space occupied by an object.
Density – the ratio of mass to volume
The density of an object is calculated by
dividing the mass of the object by its volume.
Density
The object with the lowest density will float.
Density is a Derived Unit
Combination of base units
Mass (g or kg)
Volume (m3 or cm3) length length length
D = MV
1 cm3 = 1 mL1 dm3 = 1 L
Density (kg/m3 or g/cm3 or g/mL)
mass per volume
DensityM
ass
(g)
Volume (cm3)
Δx
Δyslope D
V
M
Density
An object has a volume of 825 cm3 and a density of 13.6 g/cm3. Find its mass.
GIVEN:
V = 825 cm3
D = 13.6 g/cm3
M = ?
WORK:
M = DV
M = (13.6 g/cm3)(825cm3)
M = 11,200 g
V
MD We will talk about
how to round correctly soon!
Density
A liquid has a density of 0.87 g/mL. What volume is occupied by 25 g of the liquid?
GIVEN:
D = 0.87 g/mL
V = ?
M = 25 g
WORK:
V = M D
V = 25 g
0.87 g/mL
V = 29 mLV
MD
Dimensional Analysis –
The technique of converting between units.
Six Steps that May Help You:
1. Write down the starting number with unit, draw a horizontal line under it, and a vertical line next to it.
2. Look at the unit and write down conversion factors that might work. (You have to THINK of these.)
3. Write down UNITS for conversion factors ABOVE & BELOW the line so the unit you started with will CANCEL OUT.
4. Fill in the numbers that go with the conversion factors.
6. Do the math & don’t forget to round correctly at the end & put the correct unit on your final answer.
5. Cancel out units as required.
Example:
1. Write it down, draw the lines.
14 cm
Convert 14 cm to meters
Example:
14 cm
Convert 14 cm to meters
2. Write down the factor.
Our factor that we think will work is 100 cm = 1 meter
Example:
14 cm
Convert 14 cm to meters
Conversion Factor: 100 cm = 1 meter
3. Fill in the units from the factor so that the starting unit will cancel out.
cm
m
Example:
14 cm
Convert 14 cm to meters
Conversion Factor: 100 cm = 1 meter
cm
m
4. Fill in the numbers from the conversion factor.
100
1
Example:
14 cm
Convert 14 cm to meters
Conversion Factor: 100 cm = 1 meter
cm
m
100
1
5. Cancel out units.
Example:
14 cm
Convert 14 cm to meters
cm
m
100
1
6. Do the math.
Multiply all numbers across the top, divide by each number across the bottom.
= 14 100
= 0.14 mDon’t forgetunits!
1x
Example: Convert 4.7 kg to pounds
Some Problems Require Many Steps …Just draw a longer line!
Example:
1. Write it down, draw the lines.
4.7 kg
Convert 4.7 kg to pounds
Example:
4.7 kg
2. Write down the factors.
Our factors that we think will work are 454 g = 1 lb 1000g = 1 kg
Convert 4.7 kg to pounds
Example:
4.7 kg
Conversion Factors: 454 g = 1 lb and 1000g = 1 kg
3. Fill in the units from the factors so that the starting unit will cancel out.
kg
g
Convert 4.7 kg to pounds
g
lb
Example:
4.7 kg
4. Fill in the numbers from the conversion factors.
1
1000
Convert 4.7 kg to pounds
Conversion Factors: 454 g = 1 lb and 1000g = 1 kg
g
kg
lb
g
1
454
Example:
4.7 kg
1
1000
Convert 4.7 kg to pounds
Conversion Factors: 454 g = 1 lb and 1000g = 1 kg
g
kg
lb
g
1
454
5. Cancel out units.
*Example:
4.7 kg
1
1000
Convert 4.7 kg to pounds
g
kg
lb
g
1
454
6. Do the math.
Multiply all numbers across the top, divide by each number across the bottom.
=
=
Don’t forgetunits!
10. lb4.7 x 1000 x 1 1 454
How to measure
Mass – balance
Weight – a spring scale
Volume – volumetric flask