introduction forces
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Copyright © 2012 Pearson Education Inc.
Introduction
Forces
Physics 7C lecture A
Thursday September 26, 8:00 AM – 9:20 AMEngineering Hall 1200
Copyright © 2012 Pearson Education Inc.
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 Xia210F Rowland Hall, email: xia.jing@uci.edu
Office Hours: 9:30 AM - 10:30 AM every Thursday in my office 210F Rowland Hall
Lectures:
Tuesday/Thursday, 8:00 AM – 9:20 AM in EH 1200Discussion sessions: Wednesday
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Course information
7C Grade:
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Course information
Midterm 1 (Chapters 4, 5, 6 and 7): Thursday 8-9:20 AM, October 24, EH 1200·
Midterm 2 (Chapters 8, 9 and 10): Thursday 8-9:20 AM, November 21, EH 1200.
Final Exam (Comprehensive, with emphasis on the chapter 8 onwards): Two-hour exam on December 11th or 13th.
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 long-range 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.
1 2 3
R=F +F +F + = 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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