THE TOPICS OF PHYSICS

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The Science of Physics
THE TOPICS OF PHYSICS
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Physics is Everywhere
We are surrounded by principles of physics in our
everyday lives. In fact, most people know much
more about physics than they realize.
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The Scientific Method
Physics uses models that describe only part of
reality Models can help build hypotheses Models
help guide experimental design
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Measurements in Experiments
NUMBERS AS MEASUREMENTS Numerical measurements
are different from the numbers used in a
mathematics class. Math numbers have no
dimensions. The description of what kind of
physical quantity is represented by a certain
measurement is called dimension. In the next
several chapters, you will encounter three basic
dimensions length, mass, and time
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Many other measurements can be expressed in terms
of these three dimensions. For example,
physical quantities, such as force, velocity,
energy, volume, and acceleration, can all be
described as combinations of length, mass, and
time.
Velocity displacement / time (m/s)
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Units
The description of how much of a physical
quantity is represented by a certain numerical
measurement depends on the units with which the
quantity is measured. For example, small
distances are more easily measured in millimeters
than in kilometers.
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SI System
SI is the standard measurement system for science
M K S
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SI Prefixes
SI uses prefixes to accommodate extremes
8.25 105 meters (m) from Earths surface can
be expressed as 825 kilometers (km)
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Both dimension and units must agree Measurements
of physical quantities must be expressed in units
that match the dimensions of that quantity. For
example, measurements of length cannot be
expressed in units of kilograms because units of
kilograms describe the dimension of mass. It is
very important to be certain that a measurement
is expressed in units that refer to the correct
dimension.
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Accuracy Precision
Because theories are based on observation and
experiment, careful measurements are very
important in physics. But in reality, no
measurement is perfect. In describing the
imperfection, there are two factors to consider
a measurements accuracy and a measurements
precision.
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Accuracy - describes how close a measured value
is to the true value of the quantity
measured Problems with accuracy are due to
error Precision - refers to the degree of
exactness with which a measurement is made and
stated Precision describes the limitations of
the measuring instrument
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Accurate or Precise ?
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Accurate or Precise ?
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Accurate or Precise ?
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Accurate or Percise ?
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Significant Figures
Significant Figures help keep track of imprecision
significant figures - those digits in a
measurement that are known with certainty plus
the first digit that is uncertain
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Significant Figures
Note the number of significant figures is
determined by the precision of the markings on
the measuring scale. The last digit is reported
as a 0.2 (for the 0.2 cm past the 18 cm mark).
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Significant figures in calculations require
special rules
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Significant figures in calculations require
special rules
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Rounding
Calculators do not pay attention to significant
figures
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Order-of-Magnitude
Order-of-magnitude estimations check
answers Because the scope of physics is so wide
and the numbers may be astronomically large or
subatomically small, it is often useful to
estimate an answer to a problem before trying to
solve the problem exactly.
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This kind of estimate is called an
order-of-magnitude calculation, which means
determining the power of 10 that is closest to
the actual numerical value of the quantity.
Once you have done this, you will be in a
position to judge whether the answer you get from
a more exacting procedure is correct.
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The reason we speak of "order of magnitude" is to
quickly classify the size of a problem,
phenomenon, or thing. It doesn't have to be
physics or even science. For instance, in
gauging the cost of an object. We do not gauge
the cost of the object differently if it was 10
or 13. We do gauge it different if it were
100, 1000, 10,000 etc.
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INDEPENDENTLY estimate the order of magnitude
of the following 1. People in US 2. Students
at HHMS 3. Your age 4. A penny 5. A cell in
the human body