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Chapter 11

Chapter 11. Motion. 11.1 Distance and Direction. Choosing a frame of reference To describe motion accurately and completely, a frame of reference is necessary. Ex: A butterfly moves, but so does the Earth.

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Chapter 11

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  1. Chapter 11 Motion

  2. 11.1 Distance and Direction • Choosing a frame of reference • To describe motion accurately and completely, a frame of reference is necessary. • Ex: A butterfly moves, but so does the Earth. • Frame of reference-system of objects that are not moving with respect to one another. • How fast are you moving? • Relative Motion- Movement in relation to a frame of reference. • Ex: A train to people on a platform is moving. To people on the train, they don’t seem to be moving.

  3. Measuring Distance • Distance- The length of a path between two points. • Measuring Displacement • Displacement is the direction from the starting point and the length of a straight line from the starting point to the ending point. • Directions use displacements • Ex: Walk 5 blocks north. • Do Quick Lab on comparing distance and displacement

  4. Combining Displacement • Vector- A quantity that has a magnitude and a direction. • Magnitude can be size, length, or amount. • Vector addition is the combining of vector magnitudes and directions. You can add displacements using vector addition. • Displacement among a straight line

  5. Displacement that isn’t along a straight path • Resultant vector- The vector sum of 2 or more vectors. It shows displacement. Measuring the resultant vector (the diagonal red line) shows that the displacement from the boy's home to his school is two blocks less than the distance he actually traveled

  6. 11.2 Speed and Velocity • Speed- the ratio of the distance an object moves to the amount of time the object moves. • SI unit of speed is (m/s) meters per second

  7. Ways to express speed • Instantaneous speed- measured at a particular instant. • Instantaneous Speed, v, the rate at which an object is moving at a given moment in time. Example: A car’s speedometer.

  8. Ways to express speed • Average speed- computed for the entire duration of a trip. • speed-, v, is the total distance traveled, d, divided by the time, t, it takes to travel that distance. • Average speed = Total distance or v = d total time t

  9. Math practice • 1.A person jogs 4.0 kilometers in 32 minutes, then 2.0 kilometers in 22 minutes, and finally 1.0 kilometer in 16 minutes. What is the jogger's average speed in kilometers per minute? • 2.A train travels 190 kilometers in 3.0 hours, and then 120 kilometers in 2.0 hours. What is its average speed?

  10. Graphing Motion- Distance-Time Graphs • The slope = change in distance change in time • The slope of a line on a distance-time graph is speed. • Velocity- speed and direction on which an object is moving. Velocity is a vector. • If speed remains constant, a change in direction is a change in velocity.

  11. Combining velocities • Two or more velocities add by vector addition. • Ex: If a river flows 5 km/h east relative to the bank, and a boat travels 12 km/h east relative to the river, what is the velocity of the boat relative to the bank? • 17 km/h • Ex: If a river flows 5 km/h east relative to the bank, and a boat travels 12 km/h north relative to the river, what is the velocity of the boat relative to the bank? • You need to use Pythagorean Theorem. a2 + b2 = c2

  12. Section 11.3 Acceleration • What is acceleration? • Acceleration is the rate at which velocity changes. • Acceleration can be changes in speed, changes in direction, or changes in both. Acceleration is a vector. The basketball constantly changes velocity as it rises and falls

  13. Changes in Speed • Acceleration can be a positive change in speed (increasing), or a negative change in speed (decreasing). • (You feel acceleration in a car when you start moving after a red light.)  • As the bus moves down the street at a constant speed, its acceleration is zero. • An example of acceleration due to change in speed is free-fall. • Free-fall- The movement of an object toward Earth solely because of gravity. • Remember: Velocity is in meters per second (m/s). Acceleration is in meters per second per second (m/s2). Free-falling objects accelerate downward at 9.8 m/s2.

  14. Changes in Direction • Acceleration isn’t always the result of changes in speed. During a merry-go-round, the speed is constant but the direction changes. When you ride on a carousel, you accelerate because of the changing direction.

  15. Changes in Speed and Direction • Speed and direction changes when driving on a curved road or when riding a roller coaster. A roller coaster produces acceleration due to changes in both speed and direction

  16. acceleration • Constant Acceleration- A steady change in velocity. • Velocity of the object changes by the same amount each second. • Calculating Acceleration • You calculate acceleration for straight-line motion by dividing the change in velocity by the total time. • a = acceleration vi = initial velocity vf = final velocity t = total time • a = change in velocity a = vf- vi total time t Constant acceleration during takeoff results in changes to an aircraft's velocity that are in a constant direction

  17. Math practice • 1.A car traveling at 10 m/s starts to decelerate steadily. It comes to a complete stop in 20 seconds. What is its acceleration? • 2.An airplane travels down a runway for 4.0 seconds with an acceleration of 9.0 m/s2. What is its change in velocity during this time? • 3.A child drops a ball from a bridge. The ball strikes the water under the bridge 2.0 seconds later. What is the velocity of the ball when it strikes the water? • 4.A boy throws a rock straight up into the air. It reaches the highest point of its flight after 2.5 seconds. How fast was the rock going when it left the boy's hand?

  18. Graphs of Accelerated Motion • The slope of the speed-time graph is acceleration. • Speed-Time Graphs • Linear graph- the displayed data form straight-line parts. • Slope = acceleration.

  19. Speed-time Graphs • The horizontal part of the graph shows a biker's constant speed. The part of the graph with negative slope shows negative acceleration as the mountain biker slows to a stop.

  20. Distance-Time Graphs • Accelerated motion is represented by a curved line in a distance-time graph. A distance-time graph of accelerated motion is a curve

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