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Chapter 4Linear motion

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1) Linear means in a straight line

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2) Describe the motion of an object by its position, speed, direction and acceleration

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3) Distance is a scalar. Distance is “how much ground is covered” during motion.

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4) Displacement is a vector Displacement is X “final position – initial position”

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5) Example: distance is the path length from o to c

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Example: distance is the path length from a to c

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But displacement is the vector from a to c = final position – initial position

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6) Motion is relative. An object is moving if its position relative to a fixed point is changing.

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7) Example: A crate on a train traveling at 20 m/sec. The crate is at rest wrt the train. But the crate is moving 20 m/sec relative to the ground.

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8) Unless stated otherwise, we measure speed relative to the Earth.

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7) Speed is how fast an object is moving. Speed is a scalar quantity.

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10) From motion experiments, Galileo defined Speed = distance/time 11) Before Galileo, motion was simply described as “fast” or “slow”

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12) Instantaneous speed is the speed at any instant. 13) A speedometer shows instantaneous speed.

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11) Average speed = total distance time interval

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15) Velocity = speed in a given direction. Velocity is a vector quantity.

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16) Constant velocity = means same speed in same straight-line direction.

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17) Changing velocity : if either the speed changes or the direction changes or both change

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18) Acceleration is the rate of change of velocity acceleration is a vector. 19) acceleration = change of velocity time interval

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20) Deceleration = decrease in acceleration

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21) Accelaration for object traveling in a straight line = change in speed time interval

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Free fall

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22) projectile: an object that moves only under the influence of gravity. 23) Free fall = object falling under influence of gravity.

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24) Trajectory = path of a projectile

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25) When we talk about free fall, we ignore air resistance.

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26) Acceleration of object in free fall is about g= 10 meters/sec 2

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27) Kinematics is the study of motion Galileo discovered these relationships.

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28) There are 6 kinematics variables and 2 kinematics equations

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29)Kinematics variables for free falling object, Y = final position in y direction Y o = initial position in y direction V oy = initial velocity in y direction V fy = final velocity in y direction a y = -g t = time elapsed

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30) What is meant by “-g”? The object is accelerating in the negative direction

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31) Kinematic Equations y = y o + v o t + ½ a t 2 V = v o + at x = x o + v o t + ½ a t 2 V = v o + at

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32) For freefall, equations simplify to y = - ½ g t 2 V = - gt

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33) Rising objects: an object thrown straight up will slow down, stop and fall back down

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34) At the highest point, the vertical velocity is zero.

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28) Projectile motions 1-D 1-D 2-D

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