introduction forces

Copyright © 2012 Pearson Education Inc.

Introduction

Forces

Physics 7C lecture A

Thursday September 26, 8:00 AM – 9:20 AMEngineering Hall 1200


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)


Course information

Instructor: Jing Xia210F Rowland Hall, email: [email protected]

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


Course information

7C Grade:


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.


Course schedule


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


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


Review physics 2

• Units and physical quantities

• Motion in 1D

• Motion in 2D and 3D


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.


Unit prefixes

• Table 1.1 shows some larger and smaller units for the fundamental quantities.


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!


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|.


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.


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.


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.


Position vector

• The position vector from the origin to point P has components x, y, and z.


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.


Tranquilizing a falling monkey

• Where should the zookeeper aim?

• Follow Example 3.10.


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.


What are some properties of a force?


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.


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.


What are the magnitudes of common forces?


Drawing force vectors—Figure 4.3

• Use a vector arrow to indicate the magnitude and direction of the force.


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.


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.


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


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).

introduction forces

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