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Understanding Motion, Speed, and Acceleration: Key Concepts in Physics

This chapter introduces the fundamental concepts of motion and speed in physics. It explains how motion is defined by changes in position and the importance of reference points. Key terms like distance, displacement, velocity, and acceleration are defined and differentiated. The chapter covers average and instantaneous speed, and illustrates how velocity combines speed with direction. Additionally, it discusses calculating acceleration and how it relates to changes in velocity. Graphing techniques for analyzing motion are also explored, making this a comprehensive guide for understanding motion.

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Understanding Motion, Speed, and Acceleration: Key Concepts in Physics

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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 something move to know that motion has taken place. • You just need a reference point to know if motion occurred

  3. 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.

  4. Distance Vs Displacement Animation

  5. Speed • Rate—any change over time • Speed—the distance an object travels per unit of time. Speed = distance s = d time t • Suppose you ran 2 km in 10 min, what is your speed? s = 2 km / 10 min s = 0.2 km/min

  6. 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↑

  7. Distance vs. Time

  8. Which one is the fastest? • Which one isn’t moving?

  9. Bubble Gum Lab • Speed Challenge Lab/ wkst • http://www.nsf.gov/news/special_reports/olympics/bobsled.jsp • http://www.nsf.gov/news/special_reports/olympics/mathletes.jsp

  10. Average & Instantaneous Speed • Average speed—the total distance traveled divided by the total time of travel. • Used when speed is changing • Instantaneous speed—the speed at a given point in time. • Ex. Speedometer

  11. 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

  12. 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

  13. Acceleration • Acceleration: Change in velocity over time • acceleration = change in velocity time • When something speeds up, it has a positive acceleration • When something slows down, it has a negative acceleration • The slope of a velocity vs time graph is the acceleration • Units for acceleration are m/s/s or m/s2

  14. Changing Direction • Remember: • 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.

  15. Calculating Acceleration • As you know, • acceleration = change in velocity time • Change in velocity = final velocity (vf) – initial velocity(vi) • We can rewrite the acceleration equation as a = (vf – vi) t

  16. Calculating Positive Acceleration • An airplane starts at rest and travels down a straight runway and reaches a top speed of 80 m/s 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.

  17. Velocity vs Time Graph of Positive Acceleration

  18. Calculating Negative Acceleration • 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.

  19. Velocity vs Time Graph of Negative Acceleration

  20. Zero Acceleration • When an object has a constant velocity, it has 0 acceleration.

  21. An Object Changing Acceleration

  22. How Stuff Works: Roller Coaster & The Human Body

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