1. Speed, Velocity, and Acceleration

2. 1Both Mr Rabbit and Mr Tortoise took the same round trip, but Mr Rabbit slept & returned later.

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

4. radius = 8 km O 2A boy has been missing in a forest for 2 hours. scale = 1 cm : 5 km (a)If he walks at a speed of 4 km/h, try to locate his possible positions on the map.

5. radius = 8 km O 2A boy has been missing in a forest for 2 hours. (b)What else is important to spot the boy? The direction in which he has been walking. scale = 1 cm : 5 km

6. 1Speed How can we describe how fast an object moves? E.g.A car on Tolo Highway travels 90 km in 1 hour. We say that the car travels at a speed of 90 km/h.

7. 1Speed Speed is a measure of how fast something moves. Speed = distance traveled per unit of time SI unit: m/s or km/h (for long distances) How can we describe how fast an object moves?

8. and speeds up again to 60 km/h. aAverage speed Its average speed over the whole journey overall distance travelled total time of travel slows down to 0 km/h, A car travels at 50 km/h, 1Speed =

9. 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. aAverage speed 1Speed

10. bInstantaneous speed = speed at any instant Instantaneous speed 1Speed The word ‘speed’ alone  instantaneous speed Instantaneous speed  distance traveled in an extremely short time interval Simulation

11. Speedometer tells the car’s speed at any instant! 1Speed bInstantaneous speed

12. 2Velocity rate of change of displacement. a speed in a given direction or velocity a vector quantity direction magnitude (speed) Velocity is...

13. speed = 300 km/h direction = west Marta Train drivers concern speed only. aSpeed with direction 2Velocity Pilots concern velocity (direction & speed). speed = 90 km/h

14. bAverage velocity Average velocity = overall displacement total time of travel direction of velocity = direction of overall displacement 2Velocity

15. cInstantaneous velocity The velocity at any instant is called instantaneous velocity. If a car moves at a constant velocity... … its average and instantaneous velocities have the same value. 2Velocity

16. Q1The world record... ( ) Average speed = 10.49 = 9.53 m/s or 34.3 km/h 100 The world record of women 100-m race is 10.49 s. What is the average speed?

17. In an orienteering event, Maria and Karen reach their control points at the same time. Q2In an orienteering event... start, 10:00 am Maria, 10:30 am Karen, 10:30 am Who runs in a higher average velocity?

18. AMaria. BKaren. CUndetermined since their paths are unknown. DIncomparable since they run along different directions. Who runs in a higher average velocity? Q2In an orienteering event...

19. Note: The distance traveled is equal to magnitude of displacement only if it is a straight-line motion. Speed is usually larger than the magnitude of velocity. Q3True or false: (T/F) Average speed of an object  magnitude of its average velocity.

20. A man takes a walk starting from rest and ending at rest. Q4True or false: (T/F) It is possible for him to attain an average speed of 5 km/h but he never goes faster than 5 km/h.

21. 3Acceleration When a car moves faster and faster, its speed is increasing (velocity changed).

22. 3Acceleration When a car moves slower and slower, its speed is decreasing (velocity changed).

23. When a car changes direction, its velocity changes too. 3Acceleration

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

25. If a car accelerates at 2 m/s 2, what does that mean? 3Acceleration t = 1 s v = 2 m/s,  v = 2 m/s v = 0 t = 2 s v = 4 m/s,  v = 2 m/s v = 6 m/s,  v = 2 m/s t = 3 s 1 m t = 0 3 m 5 m

26. Airport Express takes 0.35 h to go from HK station to Airport station (34 km). Example 1 HK  Kln Kln  Tsing Yi Tsing Yi  Airport Distance between stations / km Journey time between stations / s Ave. speed between stations / km/h 2.6 8.9 (a) 153 (b) 762 (c) 90 105  Ave. speed = 34 km/0.35 h Complete table. = 97 km/h

27. Example 1 HK  Kln Kln  Tsing Yi Tsing Yi  Airport Distance between stations / km Journey time between stations / s Ave. speed between stations / km h –1 2.6 8.9 (a) 153 (b) 762 (c) 90 105 (b)Kln  Tsing Yi: Time = distance / ave. speed = 8.9 / 90 = 0.0989 h = 356 s

28. Example 1 HK  Kln Kln  Tsing Yi Tsing Yi  Airport Distance between stations / km Journey time between stations / s Ave. speed between stations / km h –1 2.6 8.9 (a) 153 (b) 762 (c) 90 105 (a)Tsing Yi  Airport: Distance = ave. speed  time = 105  12.7 762 s = (762/3600) h = 12.7 h = 22.2 km

29. Example 1 HK  Kln Kln  Tsing Yi Tsing Yi  Airport Distance between stations / km Journey time between stations / s Ave. speed between stations / km h –1 2.6 8.9 (a) 153 (b) 762 (c) 90 105 (c)HK  Kln: Ave. speed = distance / time = 2.6 / 0.0425 153 s = (153/3600) h = 0.0425 h = 61.2 km

30. A man walks from A to B at 1 km/h, A B 1 km/h Example 2 2 km/h and returns at 2 km/h. Average speed for the whole trip = ?

31. = 1.33 km/h A B 1 km/h 2 km/h Example 2 Suppose AB = 1 km Time for whole trip = = 1 h + 0.5 h = 1.5 h  whole journey = 2 km Ave. speed = distance / time = 2/1.5

32. A car travels 7 km north and then 3 km west in 10 minutes. Find C B A 3 km 7 km Example 3 (a) average speed, Ave. speed = distance travelled time taken = (7 + 3) km (10/60) h = 60 km/h

33. Example 3 A car travels 7 km north and then 3 km west in 10 minutes. Find C B A 3 km 7 km (b) ave. velocity? AC = = 7.62 km  tan  =  =23.2 o 3/7

34. Example 3 A car travels 7 km north and then 3 km west in 10 minutes. Find C B A 3 km 7 km AC = 7.62 km,  =23.2 o  Size of ave. velocity = = 45.7 km h –1 displacement time 7.62 km (10/60) h = Ave. velocity is 45.7 km h –1, 23.2° north of west. (b) ave. velocity?

35. The Ferrari 348 can go from rest to 100 km/h in 5.6 s. Example 4 What is its ave. acceleration (in m/s)? Ave. acceleration = 100 km/h 1 5.6 s (100/3.6) m/s 5.6 s = = 4.96 m/s 2

36. Q1A running student... A running student is slowing down in front of a teacher. With reference to the sign convention, Acceleration of student: positive / negative Velocity of student: positive / negative +ve

37. Quantity Unit Scalar/Vector Speed ______ _____ Velocity ______ _____ Change in velocity ______ _____ Acceleration ______ _____ Q2When time is measured... Unit of time: hour (h) km/h 1 km/h 2 scalar vector Unit of distance/displacement: kilometer (km)

38. Q3In 2.5 s, a car speeds up... In 2.5 s, a car speeds up from 60 km/h to 65 km/h... …while a bicycle goes from rest to 5 km/h. Which one has the greater acceleration? They have the same acceleration!

39. Q4A car is moving in positive... A car is moving in +ve direction. What happens if it moves under a  ve acceleration? What happens if it moves under a  ve deceleration? The car will slow down. The car will move in +ve direction with increasing speed.