 # © 2013 Pearson Education, Inc. Chapter Goal: To learn how to solve problems about motion in a straight line. Chapter 2 Kinematics in One Dimension Slide.

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© 2013 Pearson Education, Inc. Chapter Goal: To learn how to solve problems about motion in a straight line. Chapter 2 Kinematics in One Dimension Slide 2-2

© 2013 Pearson Education, Inc.  Kinematics is the name for the mathematical description of motion.  This chapter deals with motion along a straight line, i.e., runners, rockets, skiers.  The motion of an object is described by its position, velocity, and acceleration.  In one dimension, these quantities are represented by x, v x, and a x.  You learned to show these on motion diagrams in Chapter 1. Chapter 2 Preview Slide 2-3

© 2013 Pearson Education, Inc.  If you drive your car at a perfectly steady 60 mph, this means you change your position by 60 miles for every time interval of 1 hour.  Uniform motion is when equal displacements occur during any successive equal-time intervals.  Uniform motion is always along a straight line. Uniform Motion Slide 2-20 Riding steadily over level ground is a good example of uniform motion.

© 2013 Pearson Education, Inc.  An object’s motion is uniform if and only if its position-versus-time graph is a straight line.  The average velocity is the slope of the position- versus-time graph.  The SI units of velocity are m/s. Uniform Motion Slide 2-21

© 2013 Pearson Education, Inc.  The distance an object travels is a scalar quantity, independent of direction.  The displacement of an object is a vector quantity, equal to the final position minus the initial position.  An object’s speed v is scalar quantity, independent of direction.  Speed is how fast an object is going; it is always positive.  Velocity is a vector quantity that includes direction.  In one dimension the direction of velocity is specified by the  or  sign. Scalars and Vectors Slide 2-28

© 2013 Pearson Education, Inc.  An object that is speeding up or slowing down is not in uniform motion.  In this case, the position-versus-time graph is not a straight line.  We can determine the average speed v avg between any two times separated by time interval  t by finding the slope of the straight-line connection between the two points.  The instantaneous velocity is the object’s velocity at a single instant of time t.  The average velocity v avg   s/  t becomes a better and better approximation to the instantaneous velocity as  t gets smaller and smaller. Instantaneous Velocity Slide 2-31

© 2013 Pearson Education, Inc. Instantaneous Velocity Slide 2-32 Motion diagrams and position graphs of an accelerating rocket.

© 2013 Pearson Education, Inc.  As ∆t continues to get smaller, the average velocity v avg  ∆s/∆t reaches a constant or limiting value.  The instantaneous velocity at time t is the average velocity during a time interval ∆t centered on t, as ∆t approaches zero.  In calculus, this is called the derivative of s with respect to t.  Graphically, ∆s/∆t is the slope of a straight line.  In the limit ∆t  0, the straight line is tangent to the curve.  The instantaneous velocity at time t is the slope of the line that is tangent to the position-versus-time graph at time t. Instantaneous Velocity Slide 2-33

© 2013 Pearson Education, Inc.  ds/dt is called the derivative of s with respect to t.  ds/dt is the slope of the line that is tangent to the position-versus-time graph.  Consider a function u that depends on time as u  ct n, where c and n are constants:  The derivative of a constant is zero:  The derivative of a sum is the sum of the derivatives. If u and w are two separate functions of time, then: A Little Calculus: Derivatives Slide 2-46

© 2013 Pearson Education, Inc. Suppose the position of a particle as a function of time is s = 2t 2 m where t is in s. What is the particle’s velocity? Derivative Example Slide 2-47  Velocity is the derivative of s with respect to t:  The figure shows the particle’s position and velocity graphs.  The value of the velocity graph at any instant of time is the slope of the position graph at that same time.

© 2013 Pearson Education, Inc.  Suppose we know an object’s position to be s i at an initial time t i.  We also know the velocity as a function of time between t i and some later time t f.  Even if the velocity is not constant, we can divide the motion into N steps in which it is approximately constant, and compute the final position as:  The curlicue symbol is called an integral.  The expression on the right is read, “the integral of v s dt from t i to t f.” Finding Position from Velocity Slide 2-54

© 2013 Pearson Education, Inc.  The integral may be interpreted graphically as the total area enclosed between the t-axis and the velocity curve.  The total displacement ∆s is called the “area under the curve.” Finding Position From Velocity Slide 2-55

© 2013 Pearson Education, Inc. Example 2.6 The Displacement During a Drag Race Slide 2-58

© 2013 Pearson Education, Inc.  Taking the derivative of a function is equivalent to finding the slope of a graph of the function.  Similarly, evaluating an integral is equivalent to finding the area under a graph of the function.  Consider a function u that depends on time as u  ct n, where c and n are constants:  The vertical bar in the third step means the integral evaluated at t f minus the integral evaluated at t i.  The integral of a sum is the sum of the integrals. If u and w are two separate functions of time, then: A Little More Calculus: Integrals Slide 2-60

© 2013 Pearson Education, Inc.  The SI units of acceleration are (m/s)/s, or m/s 2.  It is the rate of change of velocity and measures how quickly or slowly an object’s velocity changes.  The average acceleration during a time interval ∆t is:  Graphically, a avg is the slope of a straight-line velocity- versus-time graph.  If acceleration is constant, the acceleration a s is the same as a avg.  Acceleration, like velocity, is a vector quantity and has both magnitude and direction. Motion with Constant Acceleration Slide 2-64

© 2013 Pearson Education, Inc.  Suppose we know an object’s velocity to be v is at an initial time t i.  We also know the object has a constant acceleration of a s over the time interval ∆t  t f − t i.  We can then find the object’s velocity at the later time t f as: The Kinematic Equations of Constant Acceleration Slide 2-81

© 2013 Pearson Education, Inc.  Suppose we know an object’s position to be s i at an initial time t i.  It’s constant acceleration a s is shown in graph (a).  The velocity-versus-time graph is shown in graph (b).  The final position s f is s i plus the area under the curve of v s between t i and t f : The Kinematic Equations of Constant Acceleration Slide 2-82

© 2013 Pearson Education, Inc.  Suppose we know an object’s velocity to be v is at an initial position s i.  We also know the object has a constant acceleration of a s while it travels a total displacement of ∆s  s f − s i.  We can then find the object’s velocity at the final position s f : The Kinematic Equations of Constant Acceleration Slide 2-83

© 2013 Pearson Education, Inc. The Kinematic Equations of Constant Acceleration Slide 2-84

© 2013 Pearson Education, Inc. The Kinematic Equations of Constant Acceleration Slide 2-85 Motion with constant velocity and constant acceleration. These graphs assume s i = 0, v is > 0, and (for constant acceleration) a s > 0.

© 2013 Pearson Education, Inc.  The motion of an object moving under the influence of gravity only, and no other forces, is called free fall.  Two objects dropped from the same height will, if air resistance can be neglected, hit the ground at the same time and with the same speed.  Consequently, any two objects in free fall, regardless of their mass, have the same acceleration: Free Fall Slide 2-94 In the absence of air resistance, any two objects fall at the same rate and hit the ground at the same time. The apple and feather seen here are falling in a vacuum.

© 2013 Pearson Education, Inc.  Figure (a) shows the motion diagram of an object that was released from rest and falls freely.  Figure (b) shows the object’s velocity graph.  The velocity graph is a straight line with a slope:  Where g is a positive number which is equal to 9.80 m/s 2 on the surface of the earth.  Other planets have different values of g. Free Fall Slide 2-95

© 2013 Pearson Education, Inc. Example 2.14 Finding the Height of a Leap Slide 2-98

© 2013 Pearson Education, Inc. Example 2.14 Finding the Height of a Leap Slide 2-99

© 2013 Pearson Education, Inc. Example 2.14 Finding the Height of a Leap Slide 2-100

© 2013 Pearson Education, Inc. Example 2.14 Finding the Height of a Leap Slide 2-101

© 2013 Pearson Education, Inc.  Figure (a) shows the motion diagram of an object sliding down a straight, frictionless inclined plane.  Figure (b) shows the the free-fall acceleration the object would have if the incline suddenly vanished.  This vector can be broken into two pieces: and.  The surface somehow “blocks”, so the one-dimensional acceleration along the incline is  The correct sign depends on the direction the ramp is tilted. Motion on an Inclined Plane Slide 2-102

© 2013 Pearson Education, Inc. Example 2.16 From Track to Graphs Slide 2-106

© 2013 Pearson Education, Inc. Example 2.16 From Track to Graphs Slide 2-107

© 2013 Pearson Education, Inc. Example 2.16 From Track to Graphs Slide 2-108

© 2013 Pearson Education, Inc. Example 2.16 From Track to Graphs Slide 2-109

© 2013 Pearson Education, Inc. Example 2.19 Finding Velocity from Acceleration Slide 2-114

© 2013 Pearson Education, Inc. Example 2.19 Finding Velocity from Acceleration Slide 2-115

© 2013 Pearson Education, Inc. Chapter 2 Summary Slides Slide 2-116

© 2013 Pearson Education, Inc. General Principles Slide 2-117

© 2013 Pearson Education, Inc. General Principles Slide 2-118

© 2013 Pearson Education, Inc. Important Concepts Slide 2-119

© 2013 Pearson Education, Inc. Important Concepts Slide 2-120

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