Physics and Physical Measurement
1.1.1The Realm of physics
• Measurement and uncertainties
• Scale Video
• http://www.youtube.com/watch?v=Bz9D6xba9Og&list=PL772556F1EFC4D01C&index=1&featur
e=plpp_video
• Why does that matter?
• Must have a consistent set of units agreed upon. (SI system)
SI has 7 fundamental units w/ lots of combinations.
• How far off were you?
Need an appreciation for the magnitude of things. (masses, time, distances, forces…)
1.1.2 State the ranges of magnitude of distances, masses and times that occur in the universe, from
smallest to greatest.
Distances
• Sub-nuclear particles = 10-15m
• Extent of the visible universe = 1025m
• Distance from Earth to Moon = 3.84 x 105km
• Radius of Earth = 6380km = 107m
• Diameter of a nucleus = 10-15m
• 1.1.2 State the ranges of magnitude of distances, masses and times that occur in the universe, from
smallest to greatest.
Masses
• Rest mass of and electron = 10-30kg
• The universe = 1050kg
• Mass of the Earth = 5.97 x 1024kg
• Mass of the Moon = 7.35 x 1022kg
• Mass of an electron = 9.11 x 1031kg
• 1.1.2 State the ranges of magnitude of distances, masses and times that occur in the universe, from
smallest to greatest.
Times
• Passage of light across a nucleus = 10-23s
• Age of the universe = 1018s
• Light to travel from Sun to Earth = 8min = 102s
• Period for one orbit of Earth = 365day = 107s
• Time between vibrations of Cesium = 10-10s
1.1.3 Stating ratios of quantities as differences of orders of magnitude
• 10-10m/ 10-15 m= 105
• 105 is known as a difference of five orders of magnitude.
The ratio of the diameter of the hydrogen atom to its nucleus is about 100000 times or 105 times, or a
difference of 5 orders of magnitude.
• 1.1.3 Stating ratios of quantities as differences of orders of magnitude
Example
• The rest mass of a proton is about 1.67x10-27kg. The rest mass of an electron is about 9.1x1031kg.
How many orders of magnitude bigger is the mass of the proton than that of the electron?
• 1.1.3 Stating ratios of quantities as differences of orders of magnitude
Example
• Solution
1x10-27kg and 10x10-31kg (1x10-30kg)
-27- -30 = 3 orders of magnitude
Realize that the proton is not 3 times more massive it is 103 times more massive or 1000 times more
massive.
Practice 2
• The length of a football pitch is about 100m. The distance from the earth to the moon is about
384x106m. How many orders of magnitude larger is the distance from earth to the moon than the
length of a football pitch?
• 1.1.3 Stating ratios of quantities as differences of orders of magnitude
Practice 2 solution
• Football pitch = 102
• Earth to moon = 108
• 8-2 = 6 orders of mag
Or
1,000,000 times larger
Practice 3
• The mass of the Sun is 1.99 x 1030kg. The mass of the Earth is 5.98x1024kg. How many orders of
magnitude more massive is the Sun than the Earth?
• 1.1.3 Stating ratios of quantities as differences of orders of magnitude
Practice 3 Solution
• Sun = 1030
• Earth = 1025
• 30-25 = 5 orders of magnitude
1.1.4 Estimate approximate values of everyday quantities to one or two significant figures and/or
to the nearest order of magnitude.
How high is a two storey house in meters?
What is the diameter of your pupil?
How many times does your heart beat in an hour when you are relaxed?
What is the weight of an apple in Newtons?
About 6m
about 2-4mm / 4-8mm
about 60-100 beats/min
about 1N
1.1.4 Estimate approximate values of everyday quantities to one or two significant figures and/or
to the nearest order of magnitude.
Practice 7
• Estimate the thickness of a page in your book.
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