Lecture 1 and 2
Saif Ahmed (SfA)
Lecturer, Department of Mathematics and Physics
North South University
Measurement
• We discover physics by learning how to measure the quantities
involved in physics. Among these quantities are length, time, mass,
temperature, pressure, and electric current.
• We measure each physical quantity in its own units, by comparison
with a standard. The unit is a unique name we assign to measures of
that quantity—for example, meter (m) for the quantity length. The
standard corresponds to exactly 1.0 unit of the quantity
Base Quantities
• There are so many physical quantities that it is a problem to organize them.
Fortunately, they are not all independent; for example, speed is the ratio of
a length to a time. Thus, what we do is pick out—by international
agreement—a small number of physical quantities, such as length and time,
and assign standards to them alone. We then define all other physical
quantities in terms of these base quantities and their standards (called base
standards). Speed, for example, is defined in terms of the base quantities
length and time and their base standards.
• Base standards must be both accessible and invariable. If we define the
length standard as the distance between one’s nose and the index finger on
an outstretched arm, we certainly have an accessible standard—but it will,
of course, vary from person to person. The demand for precision in science
and engineering pushes us to aim first for invariability.
The International System of Units
• In 1971, the 14th General Conference on Weights and Measures
picked seven quantities as base quantities, thereby forming the basis
of the International System of Units, abbreviated SI from its French
name and popularly known as the metric system.
Changing Units
• We often need to change the units in which a physical quantity is
expressed. We do so by a method called chain-link conversion. In this
method, we multiply the original measurement by a conversion
factor (a ratio of units that is equal to unity).
Length
• In 1792, the newborn Republic of France established a
new system of weights and measures. Its cornerstone
was the meter, defined to be one ten-millionth of the
distance from the north pole to the equator. Later, for
practical reasons, this Earth standard was abandoned
and the meter came to be defined as the distance
between two fine lines engraved near the ends of a
platinum–iridium bar, the standard meter bar, which
was kept at the International Bureau of Weights and
Measures near Paris. Accurate copies of the bar were
sent to standardizing laboratories throughout the world.
These secondary standards were used to produce other,
still more accessible standards, so that ultimately every
measuring device derived its authority from the
standard meter bar through a complicated chain of
comparisons.
Time
Time has two aspects. For civil and some scientific purposes, we want
to know the time of day so that we can order events in sequence. In
much scientific work, we want to know how long an event lasts. Thus,
any time standard must be able to answer two questions: “When did it
happen?” and “What is its duration?”
Mass
• The SI standard of mass is a cylinder of platinum
and iridium that is kept at the International bureau
of weights and measures near Paris and assigned
by international agreements, a mass of 1 kilogram.
• The masses of atoms can be compared with one
another more precisely than they can be
compared with the standard kilogram. For this
reason, we have a second mass standard. It is the
carbon-12 atom, which, by international
agreement, has been assigned a mass of 12 atomic
mass units (u)
Motion
• The world, and everything in it, moves. Even seemingly stationary
things, such as a roadway, move with Earth’s rotation, Earth’s orbit
around the Sun, the Sun’s orbit around the center of the Milky Way
galaxy, and that galaxy’s migration relative to other galaxies.
1. Motion is along a straight line
2. Forces cause motion
3. The moving object is either a particle or and object that moves like
a particle
Position and Displacement
• To locate an object means to
find its position relative to
some reference point, often
the origin (or zero point) of an
axis such as the x axis. The
positive direction of the axis is
in the direction of increasing
numbers (coordinates), which
is to the right. The opposite is
the negative direction.
Position and Displacement
• Displacement is an example of a vector quantity, which is a quantity
that has both a direction and a magnitude. (1) Its magnitude is the
distance (such as the number of meters) between the original and
final positions. (2) Its direction, from an original position to a final
position, can be represented by a plus sign or a minus sign if the
motion is along a single axis.
Average velocity
Actually, several quantities are associated with the phrase
“how fast.” One of them is the average velocity vavg, which
is the ratio of the displacement x that occurs during a
particular time interval t to that interval:
Average Speed
Average speed savg is a different way of describing “how
fast” a particle moves. Whereas the average velocity
involves the particle’s displacement x, the average
speed involves the total distance covered (for example,
the number of meters moved), independent of direction;
that is,
Example
Instantaneous Velocity and Speed
• You have now seen two ways to describe how fast something moves:
average velocity and average speed, both of which are measured over
a time interval t. However, the phrase “how fast” more commonly
refers to how fast a particle is moving at a given instant—its
instantaneous velocity (or simply velocity) v.
• The velocity at any instant is obtained from the average velocity by
shrinking the time interval t closer and closer to 0. As t dwindles,
the average velocity approaches a limiting value, which is the velocity
at that instant:
Acceleration
Constant Acceleration
In many types of motion, the acceleration is either
constant or approximately so. For example, you might
accelerate a car at an approximately constant rate when a
traffic light turns from red to green
Free Fall acceleration
• If you tossed an object either up or
down and could somehow eliminate
the effects of air on its flight, you
would find that the object
accelerates downward at a certain
constant rate. That rate is called the
free-fall acceleration, and its
magnitude is represented by g. The
acceleration is independent of the
object’s characteristics, such as
mass, density, or shape; it is the
same for all objects.
