SACE Stage 1 Conceptual Physics 2. Motion. 2.1 Motion is Relative Everything moves. Things that appear to be at rest move. Everything moves. Things that

SACE Stage 1 Conceptual SACE Stage 1 Conceptual PhysicsPhysics

2. Motion2. Motion

2.1 Motion is Relative2.1 Motion is Relative

Everything moves. Things that appear to Everything moves. Things that appear to be at rest move.be at rest move.

They move relative to the Earth, Sun and They move relative to the Earth, Sun and the stars.the stars.

A book at rest on the table is moving, it is A book at rest on the table is moving, it is moving at about 30 km/s relative to the moving at about 30 km/s relative to the sun.sun.

2.1 Motion is Relative2.1 Motion is Relative

We describe the motion of an object as We describe the motion of an object as relative to something else.relative to something else.

When a car has reached 60 km/hr, it has When a car has reached 60 km/hr, it has reached 60 km/hr relative to the ground.reached 60 km/hr relative to the ground.

Unless stated otherwise, when we Unless stated otherwise, when we describe speed in nature, assume it is describe speed in nature, assume it is relative to the ground.relative to the ground.

2.2 Speed2.2 Speed

Moving objects will cover a certain Moving objects will cover a certain distance in a given time.distance in a given time.

A car travels in km/hr. (Distance / Time)A car travels in km/hr. (Distance / Time)Speed is the measure of how fast Speed is the measure of how fast

something is moving.something is moving. It is the rate at which distance is covered.It is the rate at which distance is covered.

2.2 Speed2.2 Speed

When we see the word Rate in a statement, we When we see the word Rate in a statement, we know that something is being divided by time.know that something is being divided by time.

(s) Time

(m) Distance Speed

2.2 Speed2.2 Speed

Table 2.1 – Approximate Speeds in different units.Table 2.1 – Approximate Speeds in different units.

20 km/hr20 km/hr == 12 mi/hr12 mi/hr == 6 m/s6 m/s






2.2 Speed – Instantaneous Speed2.2 Speed – Instantaneous Speed

The speed of an object at a particular point The speed of an object at a particular point in time is called its instantaneous speed.in time is called its instantaneous speed.

2.2 Speed – Average Speed2.2 Speed – Average Speed

Average speed is the total distance covered Average speed is the total distance covered divided by the total time taken to travel that divided by the total time taken to travel that distance.distance.

(s) Time Total

(m) Covered Distance Total Speed Average

2.2 Speed – Average Speed2.2 Speed – Average Speed

Average Speed is an easy calculation.Average Speed is an easy calculation.

If it takes 4 hours to cover 240 km. What If it takes 4 hours to cover 240 km. What is your Average Speed?is your Average Speed?

2.2 Speed – Average Speed 2.2 Speed – Average Speed CalculationCalculation

m/s 16.67 Speed Average60s

min1

60min

hr1

km1

1000m

hr

km 60 Speed Average

km/hr 60 Speed Averagehrs 4

km 240 Speed Average

(s) Time Total


2.2 Speed – Question 12.2 Speed – Question 1

With the Speedometer in the dash of every With the Speedometer in the dash of every car is an odometer, which records the car is an odometer, which records the distance travelled. If the initial reading is distance travelled. If the initial reading is set to zero at the beginning of a trip and set to zero at the beginning of a trip and the reading is 35 km one-half hour later, the reading is 35 km one-half hour later, what has been your average speed?what has been your average speed?

2.2 Speed – Question 1 Answer2.2 Speed – Question 1 Answer

km/hr 70 Speed Averagehrs 0.5

km 35 Speed Average

(s) Time Total



Would it be possible to attain this average Would it be possible to attain this average speed and never exceed the 70 km/hr?speed and never exceed the 70 km/hr?


No, not if the trip started from rest. For No, not if the trip started from rest. For any period of time when the car was any period of time when the car was travelling less than 70 km/hr, there would travelling less than 70 km/hr, there would have to be periods of time where the car have to be periods of time where the car travelled greater than 70 km/hr to be able travelled greater than 70 km/hr to be able to attain an average speed of 70 km/hr.to attain an average speed of 70 km/hr.


If a cheetah can maintain a constant If a cheetah can maintain a constant speed of 25 m/s, it will cover 25m every speed of 25 m/s, it will cover 25m every second. At this rate, how far will it travel in second. At this rate, how far will it travel in 10s? 1 minute?10s? 1 minute?


When can re arrange our formula to allow us to When can re arrange our formula to allow us to work out the distance travelled.work out the distance travelled.

250m Covered Distance Total

10s m/s 25 Covered Distance Total

Time Total Speed Average Covered Distance Total

(s) Time Total


1500m Covered Distance Total

60s m/s 25 Covered Distance Total

Time Total Speed Average Covered Distance Total

2.3 Velocity2.3 Velocity

What is the difference between speed and What is the difference between speed and velocity?velocity?

Velocity is a speed in a given direction.Velocity is a speed in a given direction.A car that is travelling at 60 km/hr has A car that is travelling at 60 km/hr has

speed but when we say 60 km/hr North, it speed but when we say 60 km/hr North, it now has velocity.now has velocity.

2.3 Velocity – Question2.3 Velocity – Question

The speedometer in a car moving The speedometer in a car moving northward reads 60 km/hr. It passes northward reads 60 km/hr. It passes another car that travels at 60 km/hr another car that travels at 60 km/hr Southward. Do both cars have the same Southward. Do both cars have the same speed? Do they have the same velocity?speed? Do they have the same velocity?

2.3 Velocity – Question Answer2.3 Velocity – Question Answer

Both cars have the same speed but will Both cars have the same speed but will have opposite velocities as they are have opposite velocities as they are travelling in opposite directions.travelling in opposite directions.

2.3 Velocity – Constant Velocity2.3 Velocity – Constant Velocity

To achieve constant velocity, it is To achieve constant velocity, it is reasonable to see that an object must reasonable to see that an object must have constant speed and constant have constant speed and constant direction.direction.

Motion at constant velocity is motion in a Motion at constant velocity is motion in a straight line.straight line.

2.3 Velocity – Changing Velocity2.3 Velocity – Changing Velocity

If either the speed or direction change, If either the speed or direction change, then velocity has changed.then velocity has changed.

In a car there are three things that can In a car there are three things that can change velocity. The accelerator change velocity. The accelerator increasesincreases speed, the brake speed, the brake decreasesdecreases speed and the steering wheel speed and the steering wheel changeschanges the the direction.direction.

2.4 Acceleration2.4 Acceleration

Acceleration describes how velocity is Acceleration describes how velocity is changing.changing.

Velocity can change by changing its speed Velocity can change by changing its speed or direction.or direction.

Although the moon that orbits the Earth Although the moon that orbits the Earth isn’t changing it’s speed, it is accelerating isn’t changing it’s speed, it is accelerating as it is changing it’s direction.as it is changing it’s direction.


interval time

yin velocit ChangeonAccelerati


When driving a car, there is 3 ways to When driving a car, there is 3 ways to accelerate. accelerate.

Speed up, slow down or change direction.Speed up, slow down or change direction.

2.4 Acceleration – Question 12.4 Acceleration – Question 1

Suppose a car moving in a straight line Suppose a car moving in a straight line steadilty increases its speed each second, steadilty increases its speed each second, first from 35 – 40 km/hr, then from first from 35 – 40 km/hr, then from 40 - 45 km/hr, then from 45 – 50 km/hr. 40 - 45 km/hr, then from 45 – 50 km/hr. What is its acceleration?What is its acceleration?

2.4 Acceleration – Question 1 2.4 Acceleration – Question 1 AnswerAnswer

We see that the speed increases by 5 We see that the speed increases by 5 km/hr during each 1s interval. The km/hr during each 1s interval. The acceleration is therefore 5 km/hr each acceleration is therefore 5 km/hr each second.second.

2.4 Acceleration – Question 22.4 Acceleration – Question 2

In 5 seconds a car is moving in a straight In 5 seconds a car is moving in a straight line changes its speed from 50 km/hr to 65 line changes its speed from 50 km/hr to 65 km/hr while a truck goes from rest to 15 km/hr while a truck goes from rest to 15 km/hr in a straight line. Which undergoes km/hr in a straight line. Which undergoes the greater acceleration? What is the the greater acceleration? What is the acceleration of each vehicle.acceleration of each vehicle.

2.4 Acceleration – Question 2 2.4 Acceleration – Question 2 AnswerAnswer

The car and the truck both increase their The car and the truck both increase their speeds by 15 km/hr over the same time interval speeds by 15 km/hr over the same time interval so therefore have achieved the same so therefore have achieved the same acceleration.acceleration.

3km/hr.sonAccelerati5s

km/hr 15 on Accelerati

interval time

yin velocit ChangeonAccelerati

2.5 Free Fall2.5 Free Fall

Drop something, it falls. Does it Drop something, it falls. Does it accelerate?accelerate?

GravitationGravitation causes things to accelerate causes things to accelerate when they are dropped. when they are dropped.

If the object is dropped and air resistance If the object is dropped and air resistance has been removed, the object is said to be has been removed, the object is said to be in free fall.in free fall.


Free falling objects Free falling objects are said to be under are said to be under the influence of the influence of gravity alone.gravity alone.

Elapsed Time (s)Elapsed Time (s) Instantaneous Instantaneous speed (m/s)speed (m/s)

00 00

11 1010

22 2020

33 3030

44 4040

.. ..

.. ..

.. ..

tt 10t10t


Note that in the previous table how the speed Note that in the previous table how the speed changed each second.changed each second.

During free fall, the instantaneous speed During free fall, the instantaneous speed changed by 10 m/s. The speed gain each changed by 10 m/s. The speed gain each second is its acceleration.second is its acceleration.

210m/sonAccelerati

10m/s.sonAccelerati1s

m/s 10 on Accelerati

interval time

speedin ChangeonAccelerati

2.5 Free Fall - Question2.5 Free Fall - Question

What would the speedometer What would the speedometer reading on the falling rock shown reading on the falling rock shown be 4.5s after it drops from rest? be 4.5s after it drops from rest? How about 8s after it has How about 8s after it has dropped? 100?dropped? 100?

2.5 Free Fall – Question Answer2.5 Free Fall – Question Answer

The speedometer readings would The speedometer readings would be 45 m/s, 80 m/s and 1000 m/s, be 45 m/s, 80 m/s and 1000 m/s, respectively. You can reason this respectively. You can reason this from the table on the previous from the table on the previous slide or use the equation v=gt slide or use the equation v=gt where g = 10m/s (gravity) = where g = 10m/s (gravity) = 10m/s10m/s22..


What about its upward path?What about its upward path?During the upward motion its velocity is During the upward motion its velocity is

constantly changing so it must constantly changing so it must accelerating, even when it is momentarily accelerating, even when it is momentarily still at the top.still at the top.

Acceleration due to gravity (10 m/sAcceleration due to gravity (10 m/s22) is ) is always constant.always constant.

2.6 Free Fall: How far?2.6 Free Fall: How far?

How far something falls and how fast it falls are How far something falls and how fast it falls are 2 different things!2 different things!

After 1s, an object will fall 5m. What is its After 1s, an object will fall 5m. What is its average Speed?average Speed?

5m/sSpeed Average2

10m/s0m/sSpeed Average

2

Speed FinalSpeed Initial Speed Average

(s) Time Total


2.6 Free Fall: How far? Question 12.6 Free Fall: How far? Question 1

Consider the table on the previous slide, Consider the table on the previous slide, during the span of the second time inerval, during the span of the second time inerval, the object begins at 10 m/s and ends at 20 the object begins at 10 m/s and ends at 20 m/s. What is the m/s. What is the average speedaverage speed of the of the object during this 1s interval? What is its object during this 1s interval? What is its accelerationacceleration??


210m/s on Accelerati

1

(10m/s)-(20m/s)onAccelerati

interval time

speedin change on Accelerati

5m/s1Speed Average2

20m/s10m/sSpeed Average

2

Speed FinalSpeed Initial Speed Average

s

2.6 Free Fall: How far?2.6 Free Fall: How far?

Table 2-3 Free-Fall Table 2-3 Free-Fall distances of Object distances of Object dropped from Rest.dropped from Rest.

Elapsed Time (s)Elapsed Time (s) Distance Fallen Distance Fallen (m)(m)

00 00

11 55

22 2020

33 4545

44 8080

55 125125

.. ..

.. ..

tt ½gt½gt22


An apple drops from a tree and hits the An apple drops from a tree and hits the ground in 1 second. What is its speed ground in 1 second. What is its speed upon striking the ground? What is its upon striking the ground? What is its average speed during the 1 second? How average speed during the 1 second? How high above the ground was it when it first high above the ground was it when it first dropped?dropped?


md

ssmd

gt

v

v

s

5

1/10

d distance

average)an indicates v theabovebar the(

m/s5

2

10m/sm/s0

interval time

speed finalspeed initialv speed average

10m/sv

1m/s10v

gt vspeed

find wegfor 10m/s Using

222

1

22

1

2

2

2.7 Air Resistance and Falling 2.7 Air Resistance and Falling ObjectsObjects

Drop a heavy book and a piece of paper Drop a heavy book and a piece of paper and it is obvious that the book hits the and it is obvious that the book hits the ground first.ground first.

As the paper has less mass, the air affects As the paper has less mass, the air affects it more than the book.it more than the book.

Remove air resistance and both fall at the Remove air resistance and both fall at the same rate.same rate.

All things fall at the same rate (9.8m/sAll things fall at the same rate (9.8m/s22))

2.8 Chapter Review2.8 Chapter Review

Motion is described relative to something.Motion is described relative to something.Speed is the rate at which distance is Speed is the rate at which distance is

covered measured in units of distance covered measured in units of distance divided by time.divided by time. Instantaneous speed is the speed at any Instantaneous speed is the speed at any

instant.instant.Average speed is the total distance covered Average speed is the total distance covered

divided by the time interval.divided by the time interval.


Velocity is the speed together with the Velocity is the speed together with the direction of travel.direction of travel.Velocity is constant only when the speed and Velocity is constant only when the speed and

direction are constant (a straight line motion)direction are constant (a straight line motion)


Acceleration is the rate at which velocity is Acceleration is the rate at which velocity is changing with respect to time.changing with respect to time.An object accelerates when its speed is An object accelerates when its speed is

increasing, when its speed is decreasing increasing, when its speed is decreasing and/or its direction is changing.and/or its direction is changing.

Acceleration is measured in units of speed Acceleration is measured in units of speed divided by time.divided by time.


An object in free fall is falling under the An object in free fall is falling under the influence of gravity alone, where air influence of gravity alone, where air resistance does not affect its motion.resistance does not affect its motion.An object in free fall has a constant An object in free fall has a constant

acceleration of about 10m/sacceleration of about 10m/s22..


Important TermsImportant TermsAcceleration (2.4)Acceleration (2.4)Average speed (2.2)Average speed (2.2)Elapsed time(2.5)Elapsed time(2.5)Free Fall (2.5)Free Fall (2.5) Instantaneous Speed (2.2)Instantaneous Speed (2.2)Rate (2.1)Rate (2.1)Relative (2.1)Relative (2.1)Speed (2.2)Speed (2.2)Velocity (2.3)Velocity (2.3)

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SACE Stage 1 Conceptual Physics 2. Motion. 2.1 Motion is Relative Everything moves. Things that appear to be at rest move. Everything moves. Things that