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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?.

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describing motion
Describing Motion

Point of reference: An object or group of objects that is considered to be stationary

point of reference
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?

slide4

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?

12 1 measuring motion
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.
12 1 measuring motion1
12.1 Measuring Motion

Displacement

distance

displacement

slide8

12.1 Measuring Motion

  • A scalar is a quantity that can be completely described by one value: the magnitude (size).
slide9

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.
12 1 measuring motion3
12.1 Measuring Motion

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

12 1 measuring motion4
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.

slide12

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.

slide13

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)

slide14

Speed

Distance vs. Time

B

Distance (m)

A

Time (s)

average speed
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.

average speed1
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

slide17

Average Speed

Calculate the average speed of the car at point A and point B

Distance (m)

Time (s)

slide18

Distance(m)

Time (s)

instantaneous speed
Instantaneous speed

Speed

= speed at any instant

Instantaneous speed

The word ‘speed’ alone  instantaneous speed

Instantaneous speed

 distance travelled inan extremely short time interval

instantaneous speed1
Instantaneous speed

Speed

Speedometer tells the car’s speed at any instant!

q1 the world record

( 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 )

slide22

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 = ?

slide23

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

slide24

direction

magnitude

(speed)

12.2 Velocity

Velocity is...

rate of change of displacement or

aspeed in a givendirection.

a vector

quantity

velocity

speed with direction
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

average velocity
Average velocity

Velocity

overall distance

Average velocity =

total time of travel

direction of overall distance

Direction of velocity =

instantaneous velocity
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.

so who is faster
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

q 1 in an orienteering event
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?

q 1 in an orienteering event1
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.

example 1

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:

example 11

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

example 12
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

example 13

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

example 14

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

acceleration
Acceleration

When a car moves faster and faster,

its speed is increasing (velocitychanged).

acceleration1
Acceleration

When a car moves slower and slower,

its speed is decreasing (velocitychanged).

slide38

Acceleration

When a car changes direction,

its velocity changes too.

acceleration2

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

acceleration3
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

acceleration4

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

acceleration graph
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?

q1 a running student

+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

q2 in 2 5 s a car speeds up
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!

q3 a car is moving in a positive direction
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.

slide47
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

slide49

Distance(m)

Time (s)

example 15

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.