Copyright © 2012 Pearson Education Inc. Introduction Forces Physics 7C lecture A Thursday September 26, 8:00 AM – 9:20 AM Engineering Hall 1200

Copyright © 2012 Pearson Education Inc. Course information Class website: you can find the link in eee.uci.edu Textbook: Young & Freedman, University Physics with Modern Physics (13th edition)

Copyright © 2012 Pearson Education Inc. Course information Instructor: Jing Xia 210F Rowland Hall, 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 1200 Discussion sessions: Wednesday

Copyright © 2012 Pearson Education Inc. Course information 7C Grade:

Copyright © 2012 Pearson Education Inc. 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 Final Exam (Comprehensive, with emphasis on the chapter 8 onwards): Two-hour exam on December 11th or 13th. Exams are closed-book, closed-note.

Copyright © 2012 Pearson Education Inc. Course schedule

Copyright © 2012 Pearson Education Inc. Course information Detailed class information can be 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

Copyright © 2012 Pearson Education Inc. Review physics 2 Units and physical quantities Motion in 1D Motion in 2D and 3D

Copyright © 2012 Pearson Education Inc. 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.

Copyright © 2012 Pearson Education Inc. Unit prefixes Table 1.1 shows some larger and smaller units for the fundamental quantities.

Copyright © 2012 Pearson Education Inc. 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!

Copyright © 2012 Pearson Education Inc. 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|.   

Copyright © 2012 Pearson Education Inc. 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.

Copyright © 2012 Pearson Education Inc. 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.

Copyright © 2012 Pearson Education Inc. 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 = x 2  x 1. The average x-velocity of the particle is v av-x =  x/  t. Figure 2.1 illustrates how these quantities are related.

Copyright © 2012 Pearson Education Inc. Position vector The position vector from the origin to point P has components x, y, and z.

Copyright © 2012 Pearson Education Inc. 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: a x = 0 and a y =  g.

Copyright © 2012 Pearson Education Inc. Tranquilizing a falling monkey Where should the zookeeper aim? Follow Example 3.10.

Copyright © 2012 Pearson Education Inc. 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.

Copyright © 2012 Pearson Education Inc. What are some properties of a force?

Copyright © 2012 Pearson Education Inc. 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.

Copyright © 2012 Pearson Education Inc. 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.

Copyright © 2012 Pearson Education Inc. What are the magnitudes of common forces?

Copyright © 2012 Pearson Education Inc. Drawing force vectors—Figure 4.3 Use a vector arrow to indicate the magnitude and direction of the force.

Copyright © 2012 Pearson Education Inc. 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.

Copyright © 2012 Pearson Education Inc. Decomposing a force into its component vectors Choose perpendicular x and y axes. F x and F y are the components of a force along these axes. Use trigonometry to find these force components.

Copyright © 2012 Pearson Education Inc. 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.

Copyright © 2012 Pearson Education Inc. Superposition of forces—Example 4.1 Force vectors are most easily added using components: R x = F 1x + F 2x + F 3x + …, R y = F 1y + F 2y + F 3y + …. See Example 4.1 (which has three forces).