bab 1a - ch01
TRANSCRIPT
College Physics
Chapter 1Introduction
Fundamental Quantities and Their Dimension Length [L] Mass [M] Time [T]
other physical quantities can be constructed from these three
Units To communicate the result of a
measurement for a quantity, a unit must be defined
Defining units allows everyone to relate to the same fundamental amount
Systems of Measurement
Standardized systems agreed upon by some authority,
usually a governmental body
SI -- Systéme International agreed to in 1960 by an international
committee
Length
Units SI – meter, m [L]
Defined in terms of a meter – the distance traveled by light in a vacuum during a given time
Mass Units
SI – kilogram, kg [M]
Defined in terms of kilogram, based on a specific cylinder kept at the International Bureau of Weights and Measures
Time
Units seconds, s [T]
Defined in terms of the oscillation of radiation from a cesium atom
Approximate Values Various tables in the text show
approximate values for length, mass, and time Note the wide range of values Lengths – Table 1.1 Masses – Table 1.2 Time intervals – Table 1.3
Prefixes Prefixes correspond to powers of
10 Each prefix has a specific name Each prefix has a specific
abbreviation See table 1.4
Structure of Matter
Matter is made up of molecules the smallest division that is
identifiable as a substance
Molecules are made up of atoms correspond to elements
More structure of matter Atoms are made up of
nucleus, very dense, contains protons, positively charged, “heavy” neutrons, no charge, about same mass as
protons protons and neutrons are made up of quarks
orbited by electrons, negatively charges, “light”
fundamental particle, no structure
Structure of Matter
Dimensional Analysis Technique to check the
correctness of an equation Dimensions (length, mass, time,
combinations) can be treated as algebraic quantities add, subtract, multiply, divide
Both sides of equation must have the same dimensions
Dimensional Analysis, cont. Cannot give numerical factors: this
is its limitation Dimensions of some common
quantities are listed in Table 1.5
ExampleA shape that covers an area A and has a uniform height h has a volume V = Ah. Show that V = Ah is dimensionally correct.
3[ ] LV 2[ ] LA [ ] Lh
3 2L L L [ ] [ ][ ]V A h
V Ah
The units of volume, area and height are:
We then observe that
or
Thus, the equation is dimensionally correct.
Example
Suppose that the displacement of an object is related to time according to the expression x = Bt2. What are the dimensions of B?
2x Bt
2
xB
t
2
[ ][ ]
[ ]
xB
t
2
L
T
From
we find that
Thus, B has units of
Uncertainty in Measurements There is uncertainty in every
measurement, this uncertainty carries over through the calculations need a technique to account for this
uncertainty We will use rules for significant figures
to approximate the uncertainty in results of calculations
Significant Figures A significant figure is one that is reliably
known All non-zero digits are significant Zeros are significant when
between other non-zero digits after the decimal point and another
significant figure can be clarified by using scientific notation
Operations with Significant Figures Accuracy – number of significant figures When multiplying or dividing two or
more quantities, the number of significant figures in the final result is the same as the number of significant figures in the least accurate of the factors being combined
Operations with Significant Figures, cont. When adding or subtracting, round the
result to the smallest number of decimal places of any term in the sum
If the last digit to be dropped is less than 5, drop the digit
If the last digit dropped is greater than or equal to 5, raise the last retained digit by 1
The speed of light is now defined to be 2.99792458 × 108 m/s. Express the speed of light to (a) three significant figures, (b) five significant figures, and (c) seven significant figures.
Example
82.997 924 574 10 m sc 8
3.00 10 m s
8 2.997 9 10 m s
8 2.997 924 10 m s
(a) Rounded to 3 significant figures: c =
(b) Rounded to 5 significant figures: c =
(c) Rounded to 7 significant figures: c =
A farmer measures the perimeter of a rectangular field. The length of each long side of the rectangle is found to be 38.44 m, and the length of each short side is found to be 19.5 m. What is the perimeter of the field?
Example
38.44 m 19.5 m 38.44 m 19.5 m 115.88 m
115.9 m
19.5 m
The distance around is
but this answer must be rounded to
because the distance
carries information to only one place past the decimal.
Conversions When units are not consistent, you may
need to convert to appropriate ones Units can be treated like algebraic
quantities that can “cancel” each other See the inside of the front cover for an
extensive list of conversion factors Example:
2.5415.0 38.1
1
cmin cm
in
A rectangular building lot measures 100 ft by 150 ft. Determine the area of this lot in square meters (m2).
Example
2
4 2 1 m100 ft 150 ft 1.50 10 ft
3.281 ftA w
3 21.39 10 m
One cubic centimeter (1.0 cm3) of water has a mass of 1.0 × 10–3 kg. Determine the mass of 1.0 m3 of water.
Example
33
3
32 3 3 3
3
1.0 10 kgmass density volume 1.0 m
1.0 cm
kg 10 cm1.0 10 1.0 m 1.0 10 kg
cm 1 m
Examples of various units measuring a quantity
Order of Magnitude Approximation based on a number
of assumptions may need to modify assumptions if
more precise results are needed Order of magnitude is the power of
10 that applies
An automobile tire is rated to last for 50 000 miles. Estimate the number of revolutions the tire will make in its lifetime.
Example
750 000 mi 5280 ft mi 1 rev 8 ft 3 10 rev , or
7~10 rev
A reasonable guess for the diameter of a tire might be 2.5 ft, with a circumference of about 8 ft. Thus, the tire would make
Coordinate Systems Used to describe the position of a
point in space Coordinate system consists of
a fixed reference point called the origin
specific axes with scales and labels instructions on how to label a point
relative to the origin and the axes
Types of Coordinate Systems Cartesian Plane polar
Cartesian coordinate system
Also called rectangular coordinate system
x- and y- axes Points are labeled
(x,y)
Plane polar coordinate system Origin and
reference line are noted
Point is distance r from the origin in the direction of angle , from reference line
Points are labeled (r,)
Trigonometry Review
sin
cos
tan
opposite side
hypotenuse
adjacent side
hypotenuse
opposite side
adjacent side
More Trigonometry Pythagorean Theorem
To find an angle, you need the inverse trig function for example,
Be sure your calculator is set appropriately for degrees or radians
2 2 2r x y
1sin 0.707 45
A point is located in a polar coordinate system by the coordinates r = 2.5 m and θ = 35°. Find the x- and y-coordinates of this point, assuming that the two coordinate systems have the same origin.
Example
cos 2.5 m cos35x r 2.0 m
sin 2.5 m sin35y r 1.4 m
The x coordinate is found as
and the y coordinate
A certain corner of a room is selected as the origin of a rectangular coordinate system. If a fly is crawling on an adjacent wall at a point having coordinates (2.0, 1.0), where the units are meters, what is the distance of the fly from the corner of the room?
Example
2 22 2 = 2.0 m 1.0 m = d x y 2.2 m
The x distance out to the fly is 2.0 m and the y distance up to the fly is 1.0 m. Thus, we can use the Pythagorean theorem to find the distance from the origin to the fly as,
Express the location of the fly in example before in polar coordinates.
Example
1.0 mtan 0.5
2.0 m
y
x 1tan 0.50 27
2.2 m and 27r
The distance from the origin to the fly is r in polar coordinates, and this was found to be 2.2 m. The angle is the angle between r and the horizontal reference line (the x axis in this case). Thus, the angle can be found as
and
The polar coordinates are
Summary Equations are the tools of physics
Understand what the equations mean and how to use them
Carry through the algebra as far as possible Substitute numbers at the end
Be organized