Measurements in Physics

Historical Perspective Ancient Civilizations and Monarchies• “Cubit” local customs were used• Use of length foot to standardize measurement a king’s foot

1700’s• Scientific Renaissance. France metric system, units of 10.

1960’s • SI system is adopted worldwide

Present Day:• USA, Liberia, and Burma (or Myanmar) still use Empirical System of

Measurement (inches, feet, pounds). The United Kingdom uses both systems.

Physical Quantities

• Physical quantity: a physical property that can be measured (height, volume, temperature, and other physical properties). It is described by both a number and a unit of defined size.

61.2 kilograms number unit

SI Units

• SI units: units of measurement defined by International System of Units; standard units agreed upon by scientists

• Mass is measured in kilograms (kg), length is measured in meters (m), volume is measured in cubic meters (m3), temperature is measured in kelvins (K), and time is measured in seconds (s).

Metric System

• SI units are closely related to the more familiar metric units used in all industrialized nations of the world except the United States.

Comparing SI and Metric Units

• Some SI and Metric Units and Their Equivalents

QuantitySI Unit(Symbols)

Metric Unit(Symbol) Equivalents

Mass Kilogram (kg) Gram (g) 1 kg=1000 g = 2.205 lb

Length Meter (m) Meter (m) 1 m= 3.280 ft

Volume Cubic meter (m3)

Liter (L) 1 m3=1000L=264.2 gal

Temperature Kelvin (K) Celsius Degree (⁰C)

1K = 1⁰C1⁰C=1.8⁰F

Time Second (s) Second (s) -----

SI prefixesPrefix Symbol Meaning

(fraction form)

Meaning(decimal

form)

Example

kilo- k 1000 1000 1 kilogram (kg) = 1000 g

hecto- h 100 100 1 hectogram (hg) = 100 g

deka- da 10 10 1 decaliter (daL) = 10 L

deci- d 1_10

0.1 1 deciliter (dL) = 0.1 L

centi- c 1_100

0.01 1 centimeter (cm) = 0.01 m

milli- m 1__1000

0.001 1 milligram (mg) = 0.001g

micro- μ 1____1000 000

0.000 001 1 micrometer (μm) = 0.000 001 m

Fundamental SI Units

Fundamental SI units:• Standard units agreed upon by scientistsMass:• kilograms (kg)• A measure of an object inertia or resistance to change in motion Length:• meter (m)• Distance between two points Time:• seconds (s)• How long?

Derived Units

Derived Units:• Combination of two or more fundamental units Volume:• The cubic meter (m3)• The amount of space occupied by an object• Volume = length x width x height V = l x w x h Density:• The physical property that relates the mass of an object to its volume• Mass per unit volume: grams per cubic centimeter (g/cm3) for solids

or grams per milliliter (g/mL) for liquids• Density = mass (g) / volume (mL or cm3)

Measuring Mass Mass:• A measure of an object inertia or resistance to change in motion• A measure of the amount of matter in an object W eight:• A measure of the gravitational force that the Earth, moon, or other

large body exerts on the object.Mass and Weight:• The amount of matter (mass) in an object does not depend on

location. However, the weight of the object does depend on location.• At the same location, two objects with identical masses have identical

weights; that is, gravity pulls on both equally. Thus, the mass of an object can be determined by comparing the weight of the object to the weight of a known reference standard.

Units of Mass

Unit Equivalent

1 kilogram (kg) = 1000 grams (g) = 2.205 pounds (lb)

1 gram (g) = 0.001 kilogram (kg) = 1000 milligrams (mg) = 0.0035 27 ounce (oz)

1 milligram (mg) = 0.001 gram = 0.000 001 kilogram (kg)

1 ton (t) = 2000 pounds = 907.03 kilograms (kg)

1 pound (lb) = 16 ounces = 0.454 kilogram (kg) = 454 grams (g)

1 ounce (oz) = 0.028 35 kilogram (kg) = 28.35 grams (g)= 28,350 milligrams

Units of Mass

• The kilogram is a mass of platinum-iridium cylinder preserved at the International Bureau of Weight and Measures in France.

• It was defined as mass of one liter (1000 cubic centimeters) of water at the temperature at which it is most dense (4⁰C).

Measuring Length

Length:• Distance between two points• The meter is the standard measure of length, or distance, in

both the SI and metric systems. • One meter is 39.37 inches, or about 10% longer than one yard.• More commonly used measures of length are the centimeter

(cm; 1/100 m) and the millimeter (mm; 1/1000 m)• One centimeter is a bit less then half an inch = 0.3937 inch• One millimeter is 0.039 37 inch, or about the thickness of a

dime.

Units of Length

Units of Length

Units of Length

1 Meter:• Was defined in terms of the distance from the

North Pole to the equator. It equals approximately one ten-millionth of the distance from the North Pole to the equator.

• Equals the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second.

Units of Time

Time:• The second is the SI unit of time• Until 1956 it was defined in terms of the mean solar

day. There were 86 400 seconds per day. Second was defined as 1/86400 of the mean solar day. However, the rate of rotation of the Earth is gradually slowing.

• In 1956 the mean solar day of the year 1900 was chosen as the standard on which to base a second.

• Since 1956, the second is defined as time taken by a cesium-133 atom to make 9 192 631 770 vibrations.

Units of Volume

Volume:• The amount of space occupied by an object• The SI unit for volume is the cubic meter, m3. It is so

large that the liter (1 L = 0.001 m3 = 1 dm3) is much more commonly used.

• One liter has the volume of a cube 10cm (1 dm) on edge and is a bit larger then one U.S. quart.• Each liter is further divided into 1000 milliliters (mL),

with 1 mL the size of a cube 1 cm on edge, or 1 cm3

Units of Volume

Units of Volume