# Displacement and Velocity - Link to Unit 5 Link to Unit 5 Link to Unit ...

Μηχανική

14 Νοε 2013 (πριν από 4 χρόνια και 6 μήνες)

85 εμφανίσεις

1
-
Dimensional Motion

Topics of Physics

Physics

Mechanics

Kinematics

Dynamics

Vibrations
and Waves

Optics

Electro
-
magnetism

Thermo
-
dynamics

Relativity

Quantum

Mechanics

How

Fast?

How

Far?

How

Long?

Why?

Mechanics

Kinematics

Dynamics

Objectives

Describe

motion in terms of frame of reference,
displacement, time, and velocity.

Calculate

the displacement of an object traveling at a
known velocity for a specific time interval.

Construct

and

interpret
graphs of position versus time.

One Dimensional Motion

Frame of Reference

Are you at rest?

Just exactly how fast are you and I moving?

Displacement Concept Check

You and your dog go for a walk to the park. On the way, your dog takes
many side trips to chase squirrels or examine fire hydrants. When you arrive
at the park, do you and your dog have the same displacement?

1.
Yes

2.
No

Displacement Concept Check

You and your dog go for a walk to the park. On the way, your dog takes
many side trips to chase squirrels or examine fire hydrants. When you arrive
at the park, do you and your dog have the same displacement?

1.
Yes

2.
No

Yes, you have the same displacement. Since you and your dog had
the same initial position and the same final position, then you have
(by definition) the same displacement.

Follow
-
up:

Have you and your dog traveled the same distance?

Displacement Concept Check

Does the displacement of an object depend on the specific location of the
origin of the coordinate system?

1.
Yes

2.
No

3.
It depends on the coordinate system

10

20

30

40

50

30

40

50

60

70

10

20

30

40

50

30

40

50

60

70

Displacement Concept Check

Does the displacement of an object depend on the specific location of the
origin of the coordinate system?

1.
Yes

2.
No

3.
It depends on the coordinate system

Since the displacement is the
difference

between two coordinates, the
origin does not matter.

10

20

30

40

50

30

40

50

60

70

x

40
10
30

x

60
30
30
10

20

30

40

50

30

40

50

60

70

Displacement Concept Check

Does the odometer in a car measure distance or displacement?

1.
Distance

2.
Displacement

3.
Both

Displacement Concept Check

Does the odometer in a car measure distance or displacement?

1.
Distance

2.
Displacement

3.
Both

If you go on a long trip and then return home, your odometer does not
measure zero, but it records the total miles that you traveled.

That means
the odometer records distance.

Follow
-
up:

How would you measure displacement in your car?

Displacement

x

=
x
f

x
i

displacement = change in position = final position

initial position

Displacement

y

=
y
f

y
i

displacement = change in position = final position

initial position

y
i

y
f

Positive and Negative Displacements

Speed Concept Check

If the position of a car is zero, does its speed have to be zero?

1.
Yes

2.
No

3.
It depends on the origin

0

10

-
10

Speed Concept Check

If the position of a car is zero, does its speed have to be zero?

1.
Yes

2.
No

3.
It depends on the origin

0

10

-
10

No, the speed does not depend on position, it depends on the
change

of
position. Since we know that the displacement does not depend on the
origin of the coordinate system, an object can easily start at
x

=

3 and be
moving by the time it gets to
x

= 0.

Speed Concept Check

Does the speedometer in a car measure velocity or speed?

1.
Velocity

2.
Speed

3.
Both

4.
Neither

Speed Concept Check

Does the speedometer in a car measure velocity or speed?

1.
Velocity

2.
Speed

3.
Both

4.
Neither

The speedometer clearly measures speed, not velocity. Velocity depends
on direction (vector), but the speedometer does not care what direction
you are traveling. It only measures the magnitude of the velocity, which is
the speed.

Follow
-
up:

How would you measure velocity in your car?

Speed Concept Check

You drive for 30 minutes at 30 mi/hr and then for another 30 minutes at 50
mi/hr. What is your average speed for the whole trip?

1.

more than 40 mi/hr

2.

equal to 40 mi/hr

3.

less than 40 mi/hr

Speed Concept Check

You drive for 30 minutes at 30 mi/hr and then for another 30 minutes at 50
mi/hr. What is your average speed for the whole trip?

1.

more than 40 mi/hr

2.

equal to 40 mi/hr

3.

less than 40 mi/hr

It is 40 mi/hr in this case. Since the average speed is distance/time and
you spend the same amount of time at each speed, then your average
speed would indeed be 40 mi/hr.

Speed Concept Check

You drive 4 miles at 30 mi/hr and then another 4 miles at 50 mi/hr. What is
your average speed for the whole 8
-
mile trip?

1.

more than 40 mi/hr

2.

equal to 40 mi/hr

3.

less than 40 mi/hr

Speed Concept Check

You drive 4 miles at 30 mi/hr and then another 4 miles at 50 mi/hr. What is
your average speed for the whole 8
-
mile trip?

1.

more than 40 mi/hr

2.

equal to 40 mi/hr

3.

less than 40 mi/hr

It is not 40 mi/hr! Remember that the average speed is distance/time.
Since it takes
longer

to cover 4 miles at the slower speed, you are actually
moving at 30 mi/hr for a longer period of time. Therefore, your average
speed is closer to 30 mi/hr than it is to 50 mi/hr.

Follow
-
up:

How much further would you have to drive at 50
mi/hr in order to get back your average speed of 40 mi/hr?

Average Velocity

f i
avg
f i
x x
x
v
t t t

 
 
x
i

x
f

change in position displacement
average velocity = =
change in time time interval
x
f

x
i

0

10

-
10

Velocity vs. Speed

distance traveled
average speed =
time of travel

Graphing Velocity

f i
avg
f i
x x
x
v
t t t

 
 
x
i

x
f

x
f

x
i

time (s)

position (m)

0

5

10

15

rise
run

slope

1

5

Position Graph Concept Check

The graph of position versus time for a car is given below. What can you say
about the velocity of the car over time?

t

x

1. it speeds up all the time

2. it slows down all the time

3.

it moves at constant velocity

4. sometimes it speeds up and
sometimes it slows down

5. not really sure

Position Graph Concept Check

The graph of position versus time for a car is given below. What can you say
about the velocity of the car over time?

t

x

1. it speeds up all the time

2. it slows down all the time

3.

it moves at constant velocity

4. sometimes it speeds up and
sometimes it slows down

5. not really sure

The car moves at a constant velocity because the
x
vs.

t

plot shows a
straight line
. The slope of a straight line is constant. Remember that the
slope

of
x

versus
t

is the velocity.

Position Graph Concept Check

The graph of position versus time for a car is given below. What can you say
about the velocity of the car over time?

1. it speeds up all the time

2. it slows down all the time

3.

it moves at constant velocity

4. sometimes it speeds up and
sometimes it slows down

5. not really sure

t

x

Position Graph Concept Check

The graph of position versus time for a car is given below. What can you say
about the velocity of the car over time?

1. it speeds up all the time

2. it slows down all the time

3.

it moves at constant velocity

4. sometimes it speeds up and
sometimes it slows down

5. not really sure

The car slows down all the time because the slope of the
x
vs.

t

graph
is diminishing as time goes on. Remember that the slope of
x

vs.
t

is
the velocity. At large
t,

the value of the position
x

does not change,
indicating that the car must be at rest.

Interpreting Position Graphs

Position

Time

Interpreting Velocity Graphically,
continued

Position (m)

Time (s)

1.0

2.0

3.0

4.0

10

20

30

instantaneous
rise
slope
run
f i
f i
x x
x
v
t t t

   
 
24 m
2.0 s

12m s

Table 2

Position
-
Time Data

t (s)

x (m)

0.0

0.0

1.0

2.0

2.0

8.0

3.0

18.0

4.0

32.0