Transcript video slide - UCI Physics and Astronomy

Physics 7C lecture A
Introduction
Forces
Thursday September 22, 12:30 PM – 1:50 PM
DBH 1500
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Course information
Class website:
you can find the link in eee.uci.edu
http://www.physics.uci.edu/~xia/X-lab/Teaching.html
Textbook:
Young & Freedman, University Physics with
Modern Physics (13th edition)
Copyright © 2012 Pearson Education Inc.
Course information
Instructor: Jing Xia
210F Rowland Hall, email: [email protected]
Lecture: Tuesday/Thursday, 12:30 PM – 1:50 PM in
DBH 1500
TA: Hakimi, Sahel
[email protected]
Discussion sessions by TA:
Classical Physics Dis C1 (47251), WED 9:00 am – 9:50 am in PSCB 240
Classical Physics Dis C2 (47252), WED10:00 am – 10:50 am in SSL 152
Classical Physics Dis C3 (47253), WED11:00 am – 11:50 am in SSL 159
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Course information
7C Grade:
Final Exam
Midterm Exam #1
Midterm Exam #2
Graded Online Homework (sum all assignment points)
Quiz in weekly Discussion Sessions
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(30%)
(20%)
(20%)
(20%)
(10%)
Course information
Midterm 1 (Chapters 4, 5, 6 and 7): TBD
Midterm 2 (Chapters 8, 9 and 10): TBD
Final Exam (Comprehensive, with emphasis on
the chapter 8 onwards): Two-hour exam on
TBD
Exams are closed-book, closed-note.
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Course schedule
Copyright © 2012 Pearson Education Inc.
Course information
Detailed class information can be found @:
http://www.physics.uci.edu/~xia/X-lab/Teaching.html
there is a link in eee.uci.edu
Copyright © 2012 Pearson Education Inc.
Goals for this lecture
• Review Physics 2 concepts
• To understand the meaning of force in physics
• To view force as a vector and learn how to combine forces
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Review physics 2
• Units and physical quantities
• Motion in 1D
• Motion in 2D and 3D
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The nature of physics
• Physics is an experimental science in which
physicists seek patterns that relate the phenomena of
nature.
• The patterns are called physical theories.
• A very well established or widely used theory is
called a physical law or principle.
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Unit prefixes
• Table 1.1 shows some larger and smaller units for the
fundamental quantities.
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Uncertainty and significant figures—Figure 1.7
• The uncertainty of a measured quantity
is indicated by its number of
significant figures.
• For multiplication and division, the
answer can have no more significant
figures than the smallest number of
significant figures in the factors.
• For addition and subtraction, the
number of significant figures is
determined by the term having the
fewest digits to the right of the decimal
point.
• Refer to Table 1.2, Figure 1.8, and
Example 1.3.
• As this train mishap illustrates, even a
small percent error can have
spectacular results!
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Vectors and scalars
• A scalar quantity can be described by a single
number.
• A vector quantity has both a magnitude and a
direction in space.
• In this book, a vector quantity is represented
in

boldface italic type with an arrow over it: A.


• The magnitude of A is written as A or |A|.
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Drawing vectors—Figure 1.10
• Draw a vector as a line with an arrowhead at its tip.
• The length of the line shows the vector’s magnitude.
• The direction of the line shows the vector’s direction.
• Figure 1.10 shows equal-magnitude vectors having the same
direction and opposite directions.
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Adding two vectors graphically—Figures 1.11–1.12
• Two vectors may be added graphically using either the parallelogram
method or the head-to-tail method.
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Displacement, time, and average velocity—Figure 2.1
• A particle moving along the x-axis has a coordinate x.
• The change in the particle’s coordinate is x = x2  x1.
• The average x-velocity of the particle is vav-x = x/t.
• Figure 2.1 illustrates how these quantities are related.
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Position vector
• The position vector from the origin to point P has
components x, y, and z.
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The x and y motion are separable—Figure 3.16
• The red ball is dropped at
the same time that the
yellow ball is fired
horizontally.
• The strobe marks equal time
intervals.
• We can analyze projectile
motion as horizontal motion
with constant velocity and
vertical motion with
constant acceleration: ax = 0
and ay = g.
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Tranquilizing a falling monkey
• Where should the zookeeper aim?
• Follow Example 3.10.
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Introduction to forces
• We’ve studied motion in one, two, and three
dimensions… but what causes motion?
• This causality was first understood in the late 1600s by
Sir Isaac Newton.
• Newton formulated three laws governing moving
objects, which we call Newton’s laws of motion.
• Newton’s laws were deduced from huge amounts of
experimental evidence.
• The laws are simple to state but intricate in their
application.
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What are some properties of a force?
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There are four common types of forces
• The normal force: When an
object pushes on a surface,
the surface pushes back on
the object perpendicular to
the surface. This is a contact
force.
• Friction force: This force
occurs when a surface
resists sliding of an object
and is parallel to the surface.
Friction is a contact force.
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There are four common types of forces II
• Tension force: A pulling
force exerted on an object
by a rope or cord. This is a
contact force.
• Weight: The pull of gravity
on an object. This is a longrange force.
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What are the magnitudes of common forces?
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Drawing force vectors—Figure 4.3
• Use a vector arrow to indicate the magnitude
and direction of the force.
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Superposition of forces—Figure 4.4
• Several forces acting at a point on an object have
the same effect as their vector sum acting at the
same point.
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Decomposing a force into its component vectors
• Choose perpendicular x and y axes.
• Fx and Fy are the components of a force along these axes.
• Use trigonometry to find these force components.
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Notation for the vector sum—Figure 4.7
• The vector sum of all the forces on an object is called the
resultant of the forces or the net forces.
R= F1+F2 +F3+ = F
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Superposition of forces—Example 4.1
• Force vectors are most easily added using
components: Rx = F1x + F2x + F3x + … , Ry = F1y + F2y
+ F3y + … . See Example 4.1 (which has three forces).
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