a Vector can never be equal to a Scalar
.
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1.6 Vector Algebra
a Vector quantity can be expressed by a
• Every physical theory has a range of validity. • No theory has ever regarded as the final truth.
1.2 Idealized Models A model is a simplified version of a physical
Chapter 1 Physical Quantities
and Vectors
.
1
1.1 The nature of physics
• Physics is an experimental science. • Physicists observe the phenomena of nature and try to find patterns and principles that relate these phenomena. These patterns are called physical theories or physical laws, or principles.
ball move in a vacuum.
We ignore the earth’s rotation.
point masses
We make the weight constant.
.
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1.3 Standards and Units
Physics is an experimental science. Experiments require measurements.
The system of units used around the world is called
Time S
Length m
Mass kg
Unit prefixes: kilo-,
centi-, milli-, micro-, nano
.
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1.3 Standards and Units
Operational definition: kilogram meter second Derived unit: In other cases we define a physical quantity
by describing how to calculate it from other quantities that we can measure.
speed = distance / time
v = d / t (m/s)
An equation must always be dimensionally consistent.
Is it spherical ? It moves or spins ?
Wind Air resistance
The ball’s weight varies a little as its distance from the center of the earth changes
raised seam
time, temperature, mass, density, electric charge
But many other important quantities have a direction associated with them —— Vector quantity
A F a Vector quantity has both a magnitude and a direction
Mass —— balance —— kilogram
Distance —— ruler —— meter
Time interval —— stopwatch —— second
.
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1.3 Standards and Units
International System (SI)
To make accurate, reliable measurements, we need units of measurement that do not change and that can be duplicated by observers in various locations.
Appendix A
.
8
1.4 Uncertainty and Significant Figures
.
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1.5 Vectors and Scalars
Some physical quantities can be described completely by a single number with a unit —— Scalar quantity.
How complicated?
.4Leabharlann 1.2 Idealized Models
We neglect the size and shape of the ball
by representing it as a point object, or
particle.
We neglect air resistance by making the
system that would be too complicated to analyze in full detail.
point masses rigid bodies idealized insulators …… ……
.
3
1.2 Idealized Models
For Example: A baseball thrown through the air
Physical quantity
Number + Unit
Operational definition: Some physical quantities are so fundamental that we can define them only by describing how to measure them.