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Chapter 4 Linear Motion

Chapter 4 Linear Motion. You can describe the motion of an object by its position, speed, direction, and acceleration. I. Motion Is Relative (2.1) A. Everything moves. Even things that appear to be at rest move. 1. Motion is described by motion relative to something else.

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Chapter 4 Linear Motion

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  1. Chapter 4 Linear Motion

  2. You can describe the motion of an object by its position, speed, direction, and acceleration.

  3. I. Motion Is Relative (2.1) A. Everything moves. Even things that appear to be at rest move. 1. Motion is described by motion relative to something else. a. Relative to the sun, the center of the galaxy, etc. b. We will discus motion (things in our environment) relative to the surface of the Earth.

  4. II. Speed (4.2) A. Speed is measure of how fast something is moving (rate at which distance is covered) 1. Rate– term used to describe something divided by time. 2. Speed = Units of distance / units of time (distance covered per unit of time)

  5. 3. Common Units a. miles/hour (mi/h) b. kilometers/hour (km/h) c. meters/second (m/s) Used in physics

  6. Continental drift: 2 mm/year (1mm = one millimeter = 10-3 meters) • Walking speed: 3 miles/hour = 4.4 feet/second = 1.3 m/s (1m = 1 meter, s = seconds) • Driving speed: 60 miles/hr = 100 km/hr = 88 ft/s = 26 m/s • Orbital speed (near Earth orbit) = 8 km/s = 17,000 miles/hr • Earth's speed around the sun = 30 km/s • Speed of light = 300,000 km/s = 186,000 miles/s.

  7. B. Instantaneous Speed– the speed at any instant (What you see on a car’s speedometer)

  8. C. Average Speed– total distance covered/time interval   1. Does not indicate variations in speed over time. 2. still describes rate at which distance traveled Total distance covered Average speed = Time interval

  9. 4.2Speed think! If a cheetah can maintain a constant speed of 25 m/s, it will cover 25 meters every second. At this rate, how far will it travel in 10 seconds? In 1 minute?

  10. 4.2Speed think! If a cheetah can maintain a constant speed of 25 m/s, it will cover 25 meters every second. At this rate, how far will it travel in 10 seconds? In 1 minute? Answer:In 10 s the cheetah will cover 250 m, and in 1 min (or 60 s) it will cover 1500 m.

  11. 4.2Speed think! The speedometer in every car also has an odometer that records the distance traveled. If the odometer reads zero at the beginning of a trip and 35 km a half hour later, what is the average speed?

  12. 4.2Speed think! The speedometer in every car also has an odometer that records the distance traveled. If the odometer reads zero at the beginning of a trip and 35 km a half hour later, what is the average speed? Answer:

  13. D. Velocity (4.3) 1. Velocity and speed are often used interchangeably, but in physics are different. a. Velocity is speed in a given direction.   b. Speed is how fast object moves (direction doesnot matter)

  14. E. Constant Velocity– must have constant speed and direction 1. Object moves in straight line 2. Object’s path does not curve

  15. F. Changing Velocity 1. Velocity will change if either speed or direction changes. 2. Constant speed and constant velocity are not the same.

  16. 4.3Velocity think! The speedometer of a car moving northward reads 60 km/h. It passes another car that travels southward at 60 km/h. Do both cars have the same speed? Do they have the same velocity?

  17. 4.3Velocity think! The speedometer of a car moving northward reads 60 km/h. It passes another car that travels southward at 60 km/h. Do both cars have the same speed? Do they have the same velocity? Answer: Both cars have the same speed, but they have opposite velocities because they are moving in opposite directions.

  18. III. Acceleration (4.4) A. acceleration is the rate at which the velocity is changing 1. applies to increases as well as decreases in velocity. 2. decrease in velocity often called deceleration or negative acceleration Change of velocity Acceleration = Time interval

  19. B. Acceleration applies to changes in direction as well as speed 1. When motion is in straight line the term speed and velocity are often used interchangeably.

  20. 1. When motion is in straight line the term speed and velocity are often used interchangeably. Change of speed Acceleration along a straight line = Time interval

  21. 2. Units for acceleration a bit more complicated Change of speed Acceleration = Time interval

  22. 4.4Acceleration Accelerate in the direction of velocity–speed up

  23. 4.4Acceleration Accelerate in the direction of velocity–speed up Accelerate against velocity–slow down

  24. 4.4Acceleration Accelerate in the direction of velocity–speed up Accelerate against velocity–slow down Accelerate at an angle to velocity–change direction

  25. 4.4Acceleration think! Suppose a car moving in a straight line steadily increases its speed each second, first from 35 to 40 km/h, then from 40 to 45 km/h, then from 45 to 50 km/h. What is its acceleration?

  26. 4.4Acceleration think! Suppose a car moving in a straight line steadily increases its speed each second, first from 35 to 40 km/h, then from 40 to 45 km/h, then from 45 to 50 km/h. What is its acceleration? Answer: The speed increases by 5 km/h during each 1-s interval in a straight line. The acceleration is therefore 5 km/h•s during each interval.

  27. 4.4Acceleration think! In 5 seconds a car moving in a straight line increases its speed from 50 km/h to 65 km/h, while a truck goes from rest to 15 km/h in a straight line. Which undergoes greater acceleration? What is the acceleration of each vehicle?

  28. 4.4Acceleration think! In 5 seconds a car moving in a straight line increases its speed from 50 km/h to 65 km/h, while a truck goes from rest to 15 km/h in a straight line. Which undergoes greater acceleration? What is the acceleration of each vehicle? Answer: The car and truck both increase their speed by 15 km/h during the same time interval, so their acceleration is the same.

  29. IV. Free Fall: How Fast (4.5) A. The force of Gravity causes object to accelerate downward 1. If we disregard air resistance (air friction) then free falling objects only affected by gravity. Called free fall 2. Use letter (g) to represent gravity 3. gravity varies slightly around the Earth. Average value is about 10 m/s2 4. More accurately, g is 9.8 m/s2

  30. B. The instantaneous speed of an object falling from rest is equal to the acceleration multiplied by the amount of time it falls. Instantaneous speed = acceleration x elapsed time (v symbolizes both speed and velocity)

  31. 1. speed decreases at the same rate with an object moving upwards as it increases when moving downward 2. An object thrown upward will reach a velocity of zero when it gets to its highest point

  32. V. Free Fall: How Far (4.6) A. Relationship between distance traveled, acceleration, and velocity Rearrange and solve for t

  33. 4.5Free Fall: How Fast think! During the span of the second time interval in Table 4.2, the object begins at 10 m/s and ends at 20 m/s. What is the average speed of the object during this 1-second interval? What is its acceleration?

  34. 4.5Free Fall: How Fast think! During the span of the second time interval in Table 4.2, the object begins at 10 m/s and ends at 20 m/s. What is the average speed of the object during this 1-second interval? What is its acceleration? Answer: The average speed is 15 m/s. The acceleration is 10 m/s2.

  35. 4.5Free Fall: How Fast think! What would the speedometer reading on the falling rock be 4.5 seconds after it drops from rest? How about 8 seconds after it is dropped?

  36. 4.5Free Fall: How Fast think! What would the speedometer reading on the falling rock be 4.5 seconds after it drops from rest? How about 8 seconds after it is dropped? Answer: The speedometer readings would be 45 m/s and 80 m/s, respectively.

  37. VI. Graphs of Motion A. Equations and tables not the only way to describe relationships such as velocity and acceleration. 1. Linear relationship- e.g. speed and time a. Forms straight line curve. b. Has constant slope (direct proportion)

  38. 2. Parabolic relationship– e.g. distance versus time a. Not straight line. Curved line b. Tangent at any point gives slope at that point (slope of this curve is instantaneous speed. Remember that slope is rise/run or change in y over change in x. Distance/time = speed

  39. VII. Air Resistance and Falling Objects (4.8) A. Air resistance noticeably alters the motion of things (like feathers, paper, etc.) B. Less effect on more dense (compact) objects C. Air resistance is small enough to be neglected in most cases. 

  40. VIII. How Fast, How Far, How Quickly. How Fast Changes (4.9) A. speed and velocity-used to describe how fast something free falls from rest. equation to use:

  41. B. To specify how far the object has fallen we are talking about distance. equation to use:

  42. What is this graph telling us? Change in velocity over time = acceleration

  43. C. Acceleration– how quickly does speed or velocity change 1. Very complex concept 2. rate of a rate

  44. Constant Positive Velocity

  45. Constant Negative Velocity

  46. Positive Velocity and Positive Acceleration

  47. Positive Velocity and Negative Acceleration

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