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Chapter 12 What is motion?

Chapter 12 What is motion?. Describing Motion. Point of reference : An object or group of objects that is considered to be stationary. Point of Reference. From the man standing outside’s perspective, what is happening to the bus?.

stephanie-potter
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Chapter 12 What is motion?

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  1. Chapter 12 What is motion?

  2. Describing Motion Point of reference: An object or group of objects that is considered to be stationary

  3. Point of Reference From the man standing outside’s perspective, what is happening to the bus? From the bus driver’s perspective, what is happening to the man?

  4. Point of Reference From this driver’s perspective, is he standing still, moving forward or backwards? What about the car in his rear view mirror? The buildings in front of him?

  5. 12.1 Measuring Motion • Distance – the total length that an object has travelled. • Displacement – the distance and direction from the starting point to the ending point. Path taken is not important.

  6. 12.1 Measuring Motion Displacement distance displacement

  7. How do we accurately communicate distance and displacement? 12.1 Measuring Motion

  8. 12.1 Measuring Motion • A scalar is a quantity that can be completely described by one value: the magnitude (size).

  9. 12.1 Measuring Motion • A vector has both distance and direction. • If you walk five meters east, your displacement can be represented by a 5 cm arrow pointing to the east.

  10. 12.1 Measuring Motion Both Mr. Rabbit and Mr. Tortoise took the same round trip, but Mr. Rabbit slept & returned later.

  11. 12.1 Measuring Motion Who runs faster? No, I travelled longer distance every minute. Me, as I spent less time on the trip. Comment on their argument.

  12. Speed How can we describe how fast an object moves? E.g. Acar on Jal el Dib Highway travels 90 km in 1 hour. We say that the car travels ata speedof 90 km/h.

  13. Speed How can we describe how fast an object moves? Speed is a measure of how fastsomething moves. Speed = distance travelled per unit of time SI unit: m/s or km/h(for long distances)

  14. Speed Distance vs. Time B Distance (m) A Time (s)

  15. Average speed Speed Average speed does not tell the variations during the journey. On most trips, the speed at any instant is often different from the average speed.

  16. Average speed Speed A car travels at 50 km/h, for an hour slows down to 0 km/h, for an hour and speeds up to 60 km/h for another hour. Its average speed over the whole journey overall distance travelled 50 km + 60 km = total time of travel 3 h = 36.7 km/h

  17. Average Speed Calculate the average speed of the car at point A and point B Distance (m) Time (s)

  18. Distance(m) Time (s)

  19. Instantaneous speed Speed = speed at any instant Instantaneous speed The word ‘speed’ alone  instantaneous speed Instantaneous speed  distance travelled inan extremely short time interval

  20. Instantaneous speed Speed Speedometer tells the car’s speed at any instant!

  21. ( 100 m ) Average speed = 10.49 s Q1 The world record... The world record of women 100-m race is 10.49 s. What is the average speed? = 9.53 m/sor 34.3 km/h (9.53 m/s x 3600 s/h = 34308 m/h = 34.3 km/h )

  22. 2 km/h Q2 A man walks from A to B at 1 km/h and returns at 2 km/h. 1 km/h A B Average speed for thewhole trip = ?

  23. 2 km / h Q2 1 km/ h A B whole journey = 2 km Suppose AB = 1 km Time for whole trip = = 1 h + 0.5 h = 1.5 h Avg. speed = distance / time = 2/1.5 = 1.33 km/h

  24. direction magnitude (speed) 12.2 Velocity Velocity is... rate of change of displacement or aspeed in a givendirection. a vector quantity velocity

  25. Speed with direction Velocity A subway driver’s concern is speedonly. speed = 90 km h–1 A pilot’s concern is velocity (direction & speed). speed = 300 km/h direction = west

  26. Average velocity Velocity overall distance Average velocity = total time of travel direction of overall distance Direction of velocity =

  27. Instantaneous velocity Velocity The velocity atany instant is calledinstantaneous velocity. If a car moves at a constant velocity... … its average and instantaneous velocities have the same value.

  28. So Who is Faster? Rabbit – instantaneous velocity at the beginning and end of the race Answer? They BOTH are! Tortoise – average velocity over the whole race

  29. Q1In an orienteering event... In an orienteering event, Maria and Karen reach their control points at the same time. start, 10:00 am Maria, 10:30 am Karen, 10:30 am Who runs at a higher average velocity?

  30. Q1In an orienteering event... Who runs at a higher average velocity? A Maria. B Karen. C Undetermined since their paths are unknown. D Incomparable since they run alongdifferent directions.

  31. Jounieh  Antelias Batroun Jounieh Antelias Airport 30 15.4 Distance between cities/ km (a) 17 Travel time btw cities/ min (b) 16 Avg. speed btw cities/ km/h (c) 90 55 Example 1 A car travels from Batroun to the airport in Beirut. Use the formula, s=d/t to calculate a, b and c in the following table:

  32. 30 15.4 Distance between cities/ km (a) 17 (b) 16 Travel time btw cities/ min Avg. speed btw cities/ km/h (c) 90 55 Example 1 (a) Antelias  Airport: avg. speed time Distance = = 14.7 km = 55 km/h0.267 h Jounieh  Antelias Batroun Jounieh Antelias  Airport = (16min/60min/h) = 0.267 h

  33. Example 1 (b) Jounieh Antelias: distance/ avg. speed Time = = 15.4km/90km/h =10.3min = 0.171 h Jounieh  Antelias Batroun Jounieh Antelias  Airport Distance between stations / km 15.4 Distance between stations / km (14.7) 30 Travel time btw stations / min 17 Journey time between stations / s (b) 16 Avg. speed btw stations / km/h Ave. speed between stations / km h–1 (c) 90 55

  34. 30.0 15.4 Distance between stations / km (14.7) 17 Time between stations / min (10.3) 16 Ave. speed btw stations / km/h (c) 90 55 Example 1 (c) Batroun  Jounieh: distance/ time Avg. speed = = 30km/ 0.283h = 106 km/h Jounieh  Antelias Batroun Jounieh Antelias  Airport = (17min/60min/h) = 0.283 h

  35. Jounieh  Antelias Batroun Jounieh Antelias Airport 30 15.4 Distance between cities/ km (14.7) 17 Travel time btw cities/ min (10.3) 16 Avg. speed btw cities/ km/h (106) 90 55 Example 1 (d) What was the total average speed for the whole trip? Total distance (30+15.4+14.7)km Avg. speed = (17+10.3+16)min/60min/h Total time 60.1km = 83.3 km/h 0.722h

  36. Acceleration When a car moves faster and faster, its speed is increasing (velocitychanged).

  37. Acceleration When a car moves slower and slower, its speed is decreasing (velocitychanged).

  38. Acceleration When a car changes direction, its velocity changes too.

  39. direction speed Acceleration Acceleration measures the change in velocity Acceleration = velocity per unit time overall change in velocity = total time taken vector quantity Unit: m s–1 / s = m s–2

  40. Acceleration If a car accelerates at 2 m/s2, what does that mean? v = 0 t = 0 v = 2m/s, v = 2 m/s t = 1 s 2m v = 4m/s, v = 2 m/s t = 2 s 4m t = 3 s v = 6m/s, v = 2 m/s 6m

  41. 100 km/h 5.6 s (100/3.6) m/s = 5.6 s Acceleration The Ferrari 348 can go from rest to 100 km/hin 5.6 s. What is its avg.acceleration (in m/s2)? Avg. acceleration = 1km/h = 1000m/3600s 1km/h = 1m/3.6s = 4.96 m/s2

  42. Speed Graph

  43. Acceleration Graph 25m/s 110s 90s 45s What is: a) The acceleration between O and A? b) The acceleration between A and B? c) The acceleration between B and C?

  44. +ve Q1 A running student... A running student is slowing down in front of a teacher. With reference to the sign convention, Velocity of student: positive / negative Acceleration of student: positive / negative

  45. Q2 In 2.5 s, a car speeds up... In 2.5 s, a car speeds up from 60 km/h to 65 km/h... …while in 2.5 s, a bicycle goes from rest to 5 km/h. Which onehas the greater acceleration? They have the same acceleration!

  46. Q3 A car is moving in a positive direction... A car is moving in a +ve direction. What happens if it moves under a veacceleration? The car will slow down. What happens if it moves undera vedeceleration? The car will move in +ve direction with increasing speed.

  47. Note Unit of time: hour (h) (or s if using small numbers) Unit of distance: kilometer (km) (or m if using small numbers) Quantity Unit Scalar/Vector Speed ______ _____ Velocity ______ _____ Change in velocity______ _____ Acceleration ______ _____ km/h scalar km/h vector km/h vector km/h2 vector

  48. The End

  49. Distance(m) Time (s)

  50. Jounieh  Beirut dis. Batroun Jounieh Beirut dis. Airport 2.6 8.9 Distance between stations / km (a) 153 Travel time btw stations / s (b) 762 Avg. speed btw stations / km/h (c) 90 105 Example 1 Airport Expresstakes 0.35 h to go from Batroun to the Airport (34 km).  Avg. speed = 34 km/0.35 h =97 km/h Complete thetable.

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