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Chapter 2

Chapter 2. Motion and Speed. Sec. 1 Describing Motion. Motion occurs when an object changes position. You don’t need to see it move to know that motion has happened. You just need a reference point (text p 38)

anthony-maynard
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Chapter 2

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  1. Chapter 2 Motion and Speed

  2. Sec. 1 Describing Motion • Motion occurs when an object changes position. • You don’t need to see it move to know that motion has happened. • You just need a reference point (text p 38) • A reference point is a coordinate system in which the position of the objects is measured.

  3. Frame of reference: • https://www.youtube.com/watch?v=Y75kEf8xLxI • PSCS 8 minutes

  4. Distance and Displacement • Distance—how far an object moved • The SI unit for distance is the meter,m • Displacement—the distance and direction of an object’s change in position from the starting point.

  5. Distance and Displacement graphic • Distance vs. Displacement • http://www.bing.com/videos/search?q=distance+vs+displacement+animation&FORM=VIRE1#view=detail&mid=F95E0B662B56B1874E89F95E0B662B56B1874E89

  6. Distance can be longer than Displacement • Displacement cannot be longer than Distance • Distance and Displacement can be the same.

  7. Speed • Speed—the distance an object travels per unit of time. Speed = distance s = d time t • Ex. You ran 2 km in 10 min. What is your speed? s = 2 km s=0.2 km 10 min min

  8. Speed, Distance examples • https://www.youtube.com/watch?v=5e6QDSo_8W8

  9. Average & Instantaneous Speed • Average speed—total distance traveled divided by total travel time. • Used when speed is changing • Instantaneous speed—the speed at a given point in time. Which one does a speedometer in a car measure?

  10. Velocity • Speed describes only how fast something is moving. • Velocity—includes the speed of an object AND the direction of its motion • Ex: 40 mi/hr North or 5 m/s up

  11. Change in Velocity • Velocity can change in 2 ways • Change in speed • Change in direction Or Both • Ex: a race car has a constant speed of 100km/hr around an oval track. • Speed is constant, but velocity changes

  12. Graphing Motion • On a distance vs. time graph, time goes on the x-axis and distance goes on the y-axis. • The slope (steepness) of the line represents the speed of the object.  Distance Speed Time↓

  13. Positive Speed • Positive Speed: Moving away from the origin or reference point • Graph has a positive (upward) slope

  14. Constant, Positive Speed • Straight Line • Speed isn’t • changing

  15. Fast vs. Slow Positive Speed Slow: has a shallow, gradual slope Fast: has a steep slope

  16. Changing, Positive Speed • Graph has a curve • The slope is changing, so speed is changing Speeding Up Slowing Down

  17. Negative Speed • Negative Speed: Moving toward the origin or reference point • Graph has a negative (downward) slope Constant, Negative speed

  18. Fast vs. Slow Negative Speed Fast: steep, downward slope Slow: shallow, gradual downward slope

  19. Changing, Negative Speed

  20. Not Moving (0 speed) A horizontal line (zero slope) indicates no motion (or no change from the origin) Distance (meters) Time (seconds)

  21. Which one is the fastest? Which isn’t moving?

  22. Rearranging the speed equation • The original speed equation is s=d/t • This can be rearranged to solve for d or t. • s = d t • d = s x t (how far?) • t = d (how long?) s

  23. #1 • How long will it take a bike rider to travel 450 meters at a constant speed of 2 m/s?

  24. #2 • If a car traveled 2500 m in 20 minutes, what is the average speed of the car?

  25. #3 • If a bug travels for 50 seconds at 4 meters per second, how far does it travel?

  26. Acceleration • Acceleration: Change in velocity over time • When something speeds up, it has a positive acceleration • When something slows down, it has a negative acceleration • Units for acceleration have one distance unit and 2 time units • Example Units: m/s/s or m/s2, mi/hr/min

  27. Changing Direction • A change in velocity can be either a change in speed or a change in direction. • Any time an object changes direction, its velocity changes and it is accelerating.

  28. Calculating Acceleration • Acceleration Equation: acceleration = change in velocity time • Change in velocity = final velocity (vf) – initial velocity(vi) • We can write the acceleration equation as a=(vf – vi) t (vf-vi) at

  29. Graphing Acceleration • On a Velocity vs Time Graph,the slope of the line is the acceleration • Positive acceleration has an upward slope Positive Acceleration (speeding up) + acceleration

  30. Negative Acceleration(Slowing Down) • Negative acceleration has a downward slope - acceleration

  31. Zero Acceleration • When an object has a constant velocity, it has 0 acceleration, so the graph is a horizontal line. velocity 0 acceleration time

  32. An Object Changing Acceleration

  33. Acceleration Example #1 • An airplane starts at rest and speeds up to 80m/s at the end of the runway in 20 s. What is its acceleration? • a = (vf – vi) = (80 m/s – 0 m/s) t 20 s = 4 m/s2 • The airplane is speeding up, so the acceleration is positive.

  34. Acceleration Example #2 • A skateboarder is moving at 3 m/s and comes to a stop in 2 s. What is the acceleration? a = (vf – vi) = (0 m/s – 3 m/s) = -1.5 m/s2 t 2 s • The skateboarder slowed down, so the acceleration is negative. • Note: words like at rest and stop mean velocity = 0.

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