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Chapter 1

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  1. Chapter 1 The Study of Motion

  2. Units • We can classify almost all quantities in terms of the fundamental physical quantities: • Length L • Mass M • Time T • For example: • Speed has units L/T (miles per hour)

  3. Units, cont’d • SI (Système International) Units: • MKS: • L = meters (m) • M = kilograms (kg) • T = seconds (s) • CGS: • L = centimeters (cm) • M = grams (g or gm) • T = seconds (s)

  4. Units, cont’d • British (or Imperial) Units: • L = feet (ft) • M = slugs or pound-mass (lbm) • T = seconds (s) • We will use mostly SI but we need to know how to convert back and forth.

  5. Units, cont’d • The back of your book provides numerous conversions. Here are some: • 1 inch = 2.54 cm • 1 m = 3.281 ft • 1 mile = 5280 ft • 1 km = 0.621 mi

  6. Units, cont’d • We can use these to convert a compound unit:

  7. Converting units • Look at your original units. • Determine the units you want to have. • Find the conversion you need. • Write the conversion as a fraction that replaces the original unit with the new unit.

  8. ExampleProblem 1.1 A yacht is 20 m long. Express this length in feet.

  9. Example A yacht is 20 m long. Express this length in feet. ANSWER:

  10. Example How many liters are in a five gallon bucket? There are four quarts in a gallon.

  11. Example How many liters are in a five gallon bucket? There are four quarts in a gallon. ANSWER:

  12. Metric prefixes • Sometimes a unit is too small or too big for a particular measurement. • To overcome this, we use a prefix.

  13. Metric prefixes, cont’d

  14. Metric prefixes, cont’d • Some examples: • 1 centimeter = 10-2 meters = 0.01 m • 1 millimeter = 10-3 meters = 0.001 m • 1 kilogram = 103 grams = 1,000 g

  15. Frequency and period • We define frequency as the number of events per a given amount of time. • When an event occurs repeatedly, we say that the event is periodic. • The amount of time between events is the period.

  16. Frequency and period, cont’d • The symbols we use to represent frequency are period are: • frequency: f • period: T • They are related by

  17. Frequency and period, cont’d • The standard unit of frequency is the Hertz (Hz). • It is equivalent to 1 cycle per second.

  18. ExampleExample 1.1 A mechanical stopwatch uses a balance wheel that rotates back and forth 10 times in 2 seconds. What is the frequency of the balance wheel?

  19. ExampleExample 1.1 A mechanical stopwatch uses a balance wheel that rotates back and forth 10 times in 2 seconds. What is the frequency of the balance wheel? ANSWER:

  20. Speed • Speed is the rate of change of distance from a reference point. • It is the rate of movement. • It equals the distance something travels divided by the elapsed time.

  21. Speed, cont’d • In mathematical notation, • So we can write speed as

  22. Speed, cont’d • The symbol D is the Greek letter delta and represents the change in. • As the time interval becomes shorter and shorter, we approach the instantaneous speed.

  23. Speed, cont’d • If we know the average speed and how long something travels at that speed, we can find the distance it travels:

  24. Speed, cont’d • We say that the distance is proportional to the elapsed time: • Using the speed gives us an equality, i.e., an equal sign, so we call v the proportionality constant.

  25. Speed, cont’d • Note that speed is relative. • It depends upon what you are measuring your speed against. • Consider someone running on a ship.

  26. Speed, cont’d • If you are on the boat, she is moving at

  27. Speed, cont’d • If you are on the dock, she is moving at

  28. Example When lightning strikes, you see the flash almost immediately but the thunder typically lags behind. The speed of light is 3 × 108 m/s and the speed of sound is about 345 m/s. If the lightning flash is one mile away, how long does it take the light and sound to reach you?

  29. Example ANSWER: For the thunder: For the flash:

  30. Velocity • Velocity is the speed in a particular direction. • It tells us not only “how fast” (like speed) but also how fast in “what direction.”

  31. Velocity, cont’d • In common language, we don’t distinguish between the two. • This sets you up for confusion in a physics class. • During a weather report, you might be given the wind-speed is 15 mph from the west.

  32. Velocity, cont’d • The speed of the wind is 15 mph. • The wind is blowing in a direction from the west to the east. • So you are actually given the wind velocity.

  33. Vector addition • Quantities that convey a magnitude and a direction, like velocity, are called vectors. • We represent vectors by an arrow. • The length indicates the magnitude.

  34. Vector addition, cont’d • Consider again someone running on a ship. • If in the same directions, the vectors add.

  35. Vector addition, cont’d • Consider again someone running on a ship. • If in the opposite directions, the vectors subtract.

  36. Vector addition, cont’d • What if the vectors are in different directions?

  37. Vector addition, cont’d • The resulting velocity of the bird (from the bird’s velocity and the wind) is a combination of the magnitude and direction of each velocity.

  38. c a b Vector addition, cont’d • We can find the resulting magnitude of the Pythagorean theorem.

  39. Vector addition, cont’d • Let’s find the net speed of the bird? (Why didn’t I say net velocity?) 6 8 10

  40. Vector addition, cont’d • Here are more examples, illustrating that even if the bird flies with the same velocity, the effect of the wind can be constructive or destructive.

  41. Acceleration • Acceleration is the change in velocity divided by the elapsed time. • It measures the rate of change of velocity. • Mathematically,

  42. Acceleration, cont’d • The units are • In SI units, we might use m/s2. • For cars, we might see mph/s.

  43. Acceleration, cont’d • A common way to express acceleration is in terms of g’s. • One g is the acceleration an object experiences as it falls near the Earth’s surface: g = 9.8 m/s2. • So if you experience 2g during a collision, your acceleration was 19.6 m/s2.

  44. Acceleration, cont’d • There is an important point to realize about acceleration: It is the change in velocity.

  45. Acceleration, cont’d • Since velocity is speed and direction, there are three ways it can change: • change in speed, • change in direction, or • change in both speed & direction. • The change in direction is an important case often misunderstood.

  46. Acceleration, cont’d • If you drive through a curve with the cruise control set to 65 mph, you are accelerating. • Not because your speed changes. • But because your direction is changing. • There must be an acceleration because items on your dash go sliding around. • More on this in chapter 2.

  47. ExampleExample 1.3 A car accelerates from 20 to 25 m/s in 4 seconds as it passes a truck. What is its acceleration?

  48. ExampleExample 1.3 ANSWER: The problem gives us The acceleration is:

  49. ExampleExample 1.3 CHECK: Does this make sense? The car needs to increase its speed 5 m/s in 4 seconds. If it increased 1 m/s every second, it would only reach 24 m/s. So we should expect an answer slightly more than 1 m/s every second.

  50. ExampleExample 1.4 After a race, a runner takes 5 seconds to come to a stop from a speed of 9 m/s. Find her acceleration.