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Chapter 2 One Dimensional Kinematics Learning Objectives Position, Distance, and Displacement Average Speed and Velocity Instantaneous Velocity Acceleration Motion with Constant Acceleration Applications of the Equations of Motion Freely Falling Objects PowerPoint presentations are compiled from Walker 3 rd Edition Instructor CD-ROM and Dr. Daniel Bullock’s own resources

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Position, Distance, and Displacement Coordinate system defines position Distance total length of travel –(SI unit = meter, m) –Scalar quantity Displacement change in position –Change in position = final pos. – initial pos. – x = x f – x i –Vector quantity

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Position, Distance, and Displacement Before describing motion, you must set up a coordinate system – define an origin and a positive direction. The distance is the total length of travel; if you drive from your house to the grocery store and back, what is the total distance you traveled? Displacement is the change in position. If you drive from your house to the grocery store and then to your friend’s house, what is your total distance? What is your displacement?

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Average speed and velocity Average speed distance traveled divided by the total elapsed time –SI units, meters/second (m/s) –Scalar quantity –Always positive

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Is the average speed of the red car: A.40 mi/h B.More than 40 mi/h C.Less than 40 mi/h Average speed and velocity

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Average velocity displacement divided by the total elapsed time –SI units of m/s –Vector quantity –Can de positive or negative

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Average speed and velocity Average velocity = displacement / elapsed time What’s your average velocity if you return to your starting point? What if the runner sprints 50 m in 8 s? What if he walks back to the starting line in 40 s? Can you calculate: What is his average sprint velocity? His average walking velocity? And his average velocity for the entire trip?

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Graphical Interpretation of Average Velocity The same motion, plotted one- dimensionally and as an position vs. time (x-t) graph: Position vs time graphs give us information about: average velocity slope of a line on a x-t plot is equal to the average velocity over that interval

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Graphical Interpretation of average velocity What’s the average velocity between the intervals t = 0 s t = 3 s? Is the particle moving to the left or right? What’s the average velocity between the intervals t = 2 s t = 3 s? Is the particle moving to the left or right?

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Instantaneous Velocity Instantaneous velocity This means that we evaluate the average velocity over a shorter and shorter period of time; as that time becomes infinitesimally small, we have the instantaneous velocity. Magnitude of the instantaneous velocity is known as the instantaneous speed

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Instantaneous Velocity As t smaller, the ratio x/ t becomes constant Consider the simple case of an object with constant velocity In this case as t gets smaller the ratio remains constant

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Instantaneous velocity If we have a more complex motion This plot shows the average velocity being measured over shorter and shorter intervals. The instantaneous velocity is tangent to the curve.

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Instantaneous velocity Is the instantaneous velocity at t = 0.5 s A.Greater than B.Less than C.Or equal to the instantaneous velocity at t = 1.0 s

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Instantaneous velocity Graphical Interpretation of Average and Instantaneous Velocity Average velocity is the slope of the straight line connecting two points corresponding to a given time interval Instantaneous velocity is the slope of the tangent line at a given instant of time

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Acceleration Average acceleration the change in velocity divided by the time it took to change the velocity –SI units meters/(second · second), m/s 2 –Vector quantity –Can be positive or negative –Accelerations give rise to force

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Acceleration What does it mean to have an acceleration of 10 m/s 2 ? Time (s)Velocity (m/s)

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Acceleration Instantaneous acceleration - This means that we evaluate the average acceleration over a shorter and shorter period of time; as that time becomes infinitesimally small, we have the instantaneous acceleration. When acceleration is constant, the instantaneous and average accelerations are equal

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Graphical interpretation of Acceleration Velocity vs time (v-t) graphs give us information about: average acceleration, instantaneous acceleration average velocity slope of a line on a x-t plot is equal to the average velocity over that interval the “+” 0.25 m/s 2 means the particle’s speed is increasing by 0.25 m/s every second What does the “-” 0.5 m/s 2 mean?

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Graphical interpretation of Acceleration

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Acceleration Acceleration (increasing speed) and deceleration (decreasing speed) should not be confused with the directions of velocity and acceleration: In 1-D velocities & accelerations can be “+” or “-” depending on whether they point in the “+” or “-” direction of the coordinate system Leads to two conclusion –When the velocity & acceleration have the same sign the speed of the object increases (in this case the velocity & acceleration point in the same direction) –When the velocity & acceleration have opposite signs, the speed of the object decreases (in this case the velocity & acceleration point in opposite directions

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Acceleration Under which scenarios does the car’s speed increase? Decrease?

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Motion with constant acceleration If the acceleration is constant, the velocity changes linearly: Average velocity: v 0 = initial velocity a = acceleration t = time Can you show that this equation is dimen- sionally correct? v t

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Motion with constant acceleration Average velocity: Position as a function of time: Velocity as a function of position:

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Motion with constant acceleration The relationship between position and time follows a characteristic curve. x t 2 If the time doubles what happens to the position?

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Motion with Constant Acceleration Can you derive these equations?

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Motion with constant acceleration A park ranger driving on a back country road suddenly sees a deer “frozen” in the headlights. The ranger who is driving at 11.4 m/s, immediately applies the brakes and slows with an acceleration of 3.8 m/s 2. If the deer is 20 m from the ranger’s vehicle when the brakes are applied, how close does the ranger come to hitting the deer? How much time is needed for the ranger’s vehicle to stop?

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Motion with constant acceleration Does the velocity vary uniformly with distance?

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Motion with constant acceleration Free fall is the motion of an object subject only to the influence of gravity. The acceleration due to gravity is a constant, g.

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Motion with constant acceleration An object falling in air is subject to air resistance (and therefore is not freely falling). Free fall is the motion of an object subject only to the influences of gravity An object is in free fall as soon as it is released

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Free falling objects Free fall from rest

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Trajectory of a projectile

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Chapter 2 summary Distance: total length of travel Displacement: change in position Average speed: distance / time Average velocity: displacement / time Instantaneous velocity: average velocity measured over an infinitesimally small time

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Chapter 2 summary Instantaneous acceleration: average acceleration measured over an infinitesimally small time Average acceleration: change in velocity divided by change in time Deceleration: velocity and acceleration have opposite signs Constant acceleration: equations of motion relate position, velocity, acceleration, and time Freely falling objects: constant acceleration g = 9.81 m/s 2

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