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Describing Motion: Kinematics in One Dimension AP Physics Chapter 2 Describing Motion: Kinematics in One Dimension AP Physics Section 2-1 Reference Frames and Displacement Describing Motion: Kinematics in One Dimension Mechanics – study of motion, force, energy Kinematics – how objects move Dynamics – why objects move Translational Motion – move without rotation 2-1 Describing Motion: Kinematics in One Dimension Reference Frames (Frames of Reference) Are we moving? Compared to what? Usually with “respect to the Earth” Unless otherwise specified All other cases, must specify the frame of reference Typically done with coordinate grid and x and y axis (only x or y for 1D motion) 2-1 Describing Motion: Kinematics in One Dimension Positive – up and right Negative – down and left 2-1 Describing Motion: Kinematics in One Dimension Defining Motion Position – relative to frame of reference (x or y) Displacement – change in position (meters) Dx = x2-x1 Not distance 2-1 Describing Motion: Kinematics in One Dimension Distance vs. Displacement 2-1 Describing Motion: Kinematics in One Dimension Distance – scalar (magnitude) Displacement – vector (magnitude and direction) Must give a direction East/West, up/down 2-1 Describing Motion: Kinematics in One Dimension Distance Time Graph Gizmo 2-1 Describing Motion: Kinematics in One Dimension AP Physics Section 2-2 Average Velocity Describing Motion: Kinematics in One Dimension Average Speed – distance per unit time (scalar) Average Velocity – displacement per unit time (vector)(meters/second) Dx v Dt Dx = displacement Dt = change in time 2-2 Describing Motion: Kinematics in One Dimension Distance Time Velocity Graph Gizmo 2-2 Describing Motion: Kinematics in One Dimension AP Physics Section 2-3 Instantaneous Velocity Describing Motion: Kinematics in One Dimension Instantaneous Velocity – the average velocity during an infinitesimally short time interval Dx v lim Dt 0 Dt We will only calculate situations with constant velocity or constant acceleration Calculus is required if acceleration is not constant 2-3 Describing Motion: Kinematics in One Dimension Slope of any displacement time graph is the instantaneous velocity 2-3 Describing Motion: Kinematics in One Dimension AP Physics Section 2-4 Acceleration Describing Motion: Kinematics in One Dimension Average Acceleration – change in velocity per unit time (vector) (meters/second2) Dv v v0 a Dt t t 0 v is final velocity v0 is initial velocity (or at time 0) Sign of a indicates direction of vector Deceleration is just negative acceleration 2-4 Describing Motion: Kinematics in One Dimension Acceleration is the slope of the velocity time graph 2-4 Describing Motion: Kinematics in One Dimension AP Physics Section 2-5 Motion at Constant Acceleration Describing Motion: Kinematics in One Dimension We are limited to calculations when acceleration is a constant We will use the mathematical definition of displacement, velocity, and acceleration to derive 4 Kinematic equations. Memorize these equations – you will use them a lot 2-5 Describing Motion: Kinematics in One Dimension Assume v v0 t0 = 0, it drops out a of equations We rework the t t0 v v0 definition of a acceleration to get our first working equation t v v0 at 2-5 Describing Motion: Kinematics in One Dimension x x0 v0t 1 2 at 2 v 2 v 2ax 2 0 MEMORIZE THE BIG THREE!!!!!!! 2-5 Describing Motion: Kinematics in One Dimension The 4th equation is not found in x x0 vt your book, but is in most v v0 others x x0 t 2 x x0 2 (v v0 )t 1 2-5 Describing Motion: Kinematics in One Dimension AP Physics Section 2-6 Solving Problems Describing Motion: Kinematics in One Dimension 1. Determine what the object is your are solving for. 2. Draw a diagram. Determine the positive and negative direction for motion. 3. Write down any known quantities. 4. Think about “The Physics” of the problem. 5. Determine what equation, or combination of equations will work under theses Physics conditions. 2-6 Describing Motion: Kinematics in One Dimension 6. Make your calculations. 7. See if your answer is reasonable. 8. Determine what units belong with the number, and what the direction should be if it is a vector. 2-6 Describing Motion: Kinematics in One Dimension A car slows down uniformly from a speed of 21.0 m/s to rest in 6.00s. How far did it travel in this time? 1. Object - _____________________ 2. Diagram 2-6 Describing Motion: Kinematics in One Dimension A car slows down uniformly from a speed of 21.0 m/s to rest in 6.00s. How far did it travel in this time? 1. Object – car 2. Diagram 3. Know 4. Find? v 0= v= t= 2-6 Describing Motion: Kinematics in One Dimension A car slows down uniformly from a speed of 21.0 m/s to rest in 6.00s. How far did it travel in this time? Choose your equation(s) & solve: 2-6 Describing Motion: Kinematics in One Dimension AP Physics Section 2-7 Falling Objects Describing Motion: Kinematics in One Dimension We will ignore air friction We will learn the why later. Acceleration due to gravity at earths surface is 9.80 m/s2 directed downward (-9.80m/s2) Symbol g represents acceleration due to gravity Still use motion equations but x is replaced with y a is replaced with g 2-7 Describing Motion: Kinematics in One Dimension Common Misconceptions 1. Acceleration and velocity are always in the same direction a. No, as an object is thrown upward, velocity is +y, acceleration is –y 2. Acceleration is zero at the highest point. a. No, at the highest point, the velocity is zero, but acceleration is always -9.80m/s2 b. The object changes velocity, it must have an acceleration 2-7

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posted: | 9/11/2012 |

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