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Chapter 1 Lecture Kinematics: Motion in One Dimension Week 2 Day 1 © 2014 Pearson Education, Inc.

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What is motion? Motion is a change in an object's position relative to a given observer during a certain change in time. Without identifying the observer, it is impossible to say whether the object of interest moved. Physicists say motion is relative, meaning that the motion of any object of interest depends on the point of view of the observer. © 2014 Pearson Education, Inc.

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Significant digits We should be able to measure the person's location at one instant of time to within about 0.1m but not to 0.01 m. The locations can reasonably be given as +3.0 m, which implies an accuracy of ±0.1 m. © 2014 Pearson Education, Inc.

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Video of bowling ball motion and sandbags What do you notice about the timing of the sandbags? How can you tell? © 2014 Pearson Education, Inc.

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Video of bowling ball motion and sandbags Describe the 1st two motions of the bowling ball and draw the patterns of the sandbags. –How can you tell what is happening? How are the placement of the sandbags related to the motion(s) of the bowling ball? What can you determine? How would the sandbag pattern change if the ball was initially moving faster? Slower? © 2014 Pearson Education, Inc.

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Video of bowling ball motion and sandbags Describe the 3rd and 4th motions of the bowling ball and draw the patterns of the sandbags. –How can you tell what is happening? –How are the placement of the sandbags related to the motion(s) of the bowling ball? What can you determine? How would the sandbags change if the ball was initially moving faster? Slower? The other direction? © 2014 Pearson Education, Inc.

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Video of bowling ball motion and sandbags How can the beanbag pattern tell you if an object is moving at constant speed, speeding up, or slowing down? Draw an example of each. © 2014 Pearson Education, Inc.

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Motion diagrams contain: Dots representing the location of the object for progressive, equal time intervals Velocity vectors on each dot representing the velocity of the object at each time interval (the length of the velocity vector represents how fast the object is moving) Velocity change arrows showing how the velocity vectors are changing The specified location of the observer © 2014 Pearson Education, Inc.

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Patterns found from motion diagrams The spacing of the dots allows us to visualize motion. When the object travels without speeding up or slowing down, the dots are evenly spaced. When the object slows down, the dots get closer together. When the object moves faster and faster, the dots get farther apart. © 2014 Pearson Education, Inc.

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Velocity change arrows Delta means "change in." Change always means final minus initial: what it is now compared to what it was before. The arrow above v reminds us that velocity is a vector; it has both magnitude and direction. © 2014 Pearson Education, Inc.

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Finding velocity change arrows Consider two adjacent velocity vectors, in this example at points 2 and 3. Find which vector would need to be added to the velocity corresponding to point 2 to get the velocity corresponding to point 3. © 2014 Pearson Education, Inc.

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Constructing a motion diagram © 2014 Pearson Education, Inc.

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Motion Diagram Rules: Draw the dots Draw arrows to connect the dots Draw dashed lines to separate overall motion into intervals of regular motion (motion intervals) Draw Delta v diagram for each motion interval and turn around point Draw Delta v vectors for each motion interval and turn around point © 2014 Pearson Education, Inc.

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Motion Diagram Example: A car starts at rest at a traffic light and speeds up for 4 seconds until it reaches the speed limit The car stays at the speed limit for 5 seconds Then the car slows to a stop in 2 sec at an intersection where the car has to wait 1 sec for the light to turn green A cyclist passes the car as it starts moving at constant speed (neither speeding up nor slowing down) and reaches the 2 nd intersection just when the light turns green Draw the motion diagram © 2014 Pearson Education, Inc.

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Linear motion Linear motion is a model of motion that assumes that an object, considered as a point-like object, moves along a straight line. A car moving along a straight highway can be modeled with linear motion; we simplify the car as a point, which is small compared to the length of the road. A tire of the car cannot be modeled with linear motion. © 2014 Pearson Education, Inc.

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Quantities for describing motion Motion diagrams represent motion qualitatively. To analyze situations, we need to describe motion quantitatively. These quantities are needed to describe linear motion: –Time and time interval –Position, displacement, distance, and path length –Scalar component of displacement for motion along one axis © 2014 Pearson Education, Inc.

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Reference frames require: An object of reference (or a point on an object if the object is large) A coordinate system with a scale for measuring distance A clock to measure time © 2014 Pearson Education, Inc.

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Time and time interval The time t is a clock reading. The time interval (t 2 - t 1 ) or Δt is a difference in clock readings. (The symbol delta represents "change in" and is the final value minus the initial value.) These are both scalar quantities. The SI units for both quantities are seconds (s). © 2014 Pearson Education, Inc.

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Position, displacement, distance, and path length These quantities describe the location and motion of an object. –Position is an object's location with respect to a particular coordinate system. –Displacement is a vector that starts from an object's initial position and ends at its final position. –Distance is the magnitude (length) of the displacement vector. –Path length is how far the object moved as it traveled from its initial position to its final position. Imagine laying a string along the path the object took. The length of the string is the path length. © 2014 Pearson Education, Inc.

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Summary © 2014 Pearson Education, Inc.

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Summary © 2014 Pearson Education, Inc.

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Summary © 2014 Pearson Education, Inc.

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Where can I find the lecture slides? There will be a link from MOODLE but the files can be accessed directly here: www.phys.unsw.edu.au/~jkw/phys1121_31.

Where can I find the lecture slides? There will be a link from MOODLE but the files can be accessed directly here: www.phys.unsw.edu.au/~jkw/phys1121_31.

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