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

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. Comment on their argument.
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 E.g. A car on Jal el Dib Highway travels 90 km in 1 hour.
How can we describe how fast an object moves?
E.g. A car on Jal el Dib Highway travels 90 km in 1 hour.
We say that the car travels at a speed of 90 km/h.

13. Speed = distance travelled per unit of time
How can we describe how fast an object moves?
Speed is a measure of how fast something 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. Speed
Average 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. Speed Average 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. Speed Instantaneous speed = speed at any instant
The word ‘speed’ alone  instantaneous speed
Instantaneous speed
 distance travelled in an extremely short time interval

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

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

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

23. Avg. speed = distance / timeQ2
1 km / h A B
2 km / h
Suppose AB = 1 km
 whole journey = 2 km
Time for whole trip =
= 1 h h = 1.5 h
Avg. speed = distance / time
= 1.33 km/h
= 2/1.5

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

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

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

27. Instantaneous 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.

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

29. Q1 In 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. Q1 In 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 along different directions.

31. 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:
Batroun Jounieh
Jounieh  Antelias
Antelias Airport
Distance between cities/ km
Travel time btw cities/ min
Avg. speed btw cities/ km/h 30
15.4
(a) 17
(b) 16
(c) 90 55

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

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

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

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

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

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

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

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

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

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

42. Speed Graph

43. Acceleration Graph What is: a) The acceleration between O and A?
25m/s
110s
45s
90s
(25m/s)/45s = 0.56m/s2
(-25m/s)/20s = -1.25m/s2
What is:
a) The acceleration between O and A?
b) The acceleration between A and B?
c) The acceleration between B and C?

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

45. They have the same acceleration!
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 one has 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 ve acceleration?
The car will slow down.
What happens if it moves under a ve deceleration?
The car will move in +ve direction with increasing speed.

47. Note Unit of time: hour (h) Unit of distance: kilometer (km)
(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. Example 1
Airport Express takes 0.35 h to go from Batroun to the Airport (34 km).
 Avg. speed =
34 km/0.35 h
= 97 km/h
Batroun Jounieh
Jounieh  Beirut dis.
Beirut dis. Airport
Distance between stations / km
Travel time btw stations / s
Avg. speed btw stations / km/h
2.6
8.9
(a)
153
(b)
762
(c) 90
105
Complete the table.