Unit 2
Describing Motion
© 2001-2007 Shannon W. Helzer. All Rights Reserved.
1
Units
 What is a unit?
 What are some common units that you deal with?
 What does a unit do?
 Give us a unit of volume. Mass. Speed.
© 2001-2007 Shannon W. Helzer. All Rights Reserved.
2
Speed
 What is speed?
 How fast something is going.
 i.e. the speed limit
© 2001-2007 Shannon W. Helzer. All Rights Reserved.
3
Average Speed
 What is average speed?
 The total distance traveled divided by the time of travel.
 A unit of length divided by a unit of time.
 If x = distance & t = time, then the equation for average
speed, v, is
x2  x1 x
v

t2  t1 t
© 2001-2007 Shannon W. Helzer. All Rights Reserved.
4
Units of Speed
 What are the units of speed?
 A unit of length divided by a unit of time.
 Some common units of speed are
© 2001-2007 Shannon W. Helzer. All Rights Reserved.
5
Unit Conversions
 Here is a good format for doing unit conversions.
 Convert 64 km/h to miles/s.
 km  1m ile   1h  0.011 miles
 64 


s
h  1.609km   3600s 

Practice conversions by verifying the results
given.
© 2001-2007 Shannon W. Helzer. All Rights Reserved.
6
Unit Conversions
 What happens if you have one unit of speed and want to
determine the speed in different units?
 You must convert.
 Suppose you drove to Canada and were pulled over for
speeding by a mounty. She wrote you a ticket for going 64
km/h in a 60 km/h zone. You do not think that you were
speeding because you were only going 40 MPH. Were
you speeding?
 40 MPH = 64 km/h: therefore, you were speeding!
© 2001-2007 Shannon W. Helzer. All Rights Reserved.
7
Instantaneous Speed
 The rate at which distance is being covered at that instant
in time.
 The average speed computed for a very, very short period
of time.
Speed v. Time
 At what instant was
Speed km/h)
the speed the fastest?
What was the speed?
 18 min @ 27 km/h.
 Slowest? Speed?
 30 min @ 4.5 km/h
30
20
10
10
© 2001-2007 Shannon W. Helzer. All Rights Reserved.
20
30
Time (min)
40
8
Instantaneous Speed
 Suppose you raced your 4-wheeler
over a 20 mile long track.
 If you completed the race in 40
minutes, then what was your
average speed?
 30 mph.
 Watch the animation below. Was
your speed at every instant equal
to your average speed?
 Why or why not?
x2  x1 x
v

t2  t1 t
© 2001-2007 Shannon W. Helzer. All Rights Reserved.
9
Kinematics Examples
 A MedFlight helicopter takes
off vertically with an upward
acceleration of 2.5 m/s2.
 How fast is it going in 4.5 s?
 How high up is it in 10.0 s?
v
a
t
© 2001-2007 Shannon W. Helzer. All Rights Reserved.
d
1 2
at
2
v  at
10
Kinematics Examples
 Dr. Physics had to make an emergency stop while riding his
motorcycle.
 What was his final speed?
 Initially, he was going 22.0 m/s.
 If it took him 3.5 s to stop, then what was his acceleration?
a
© 2001-2007 Shannon W. Helzer. All Rights Reserved.
v
t
d
1 2
at
2
v  at
11
Distance V. Time Graphs (WS 5 1-7)
 Let us suppose that the graph to the right
Rise d d 2  d1
v


Run t
t 2  t1
© 2001-2007 Shannon W. Helzer. All Rights Reserved.
4.5
4
3.5
Distance (m)
represents a trip taken by a physics
student in a rocket powered wheel chair.
 Answer the questions on your work sheet
using the graph provided.
 Please note that the slope of a D v. T
graph is the velocity of the moving body.
Distance v. Tim e
3
2.5
2
1.5
1
0.5
0
0
2
4
6
8
10
Tim e (s)
12
Distance V. Time Graphs (WS 5 8-12)
 Stewart Little ventures out from the
Rise d d 2  d1
v


Run t
t 2  t1
© 2001-2007 Shannon W. Helzer. All Rights Reserved.
Distance v. Tim e
450
400
350
Distance (km)
safety of the Little residence and takes
a very long trip in order to see his
girlbird Margalow.
 His trip is graphed in the graph to the
right.
 Answer the questions on your
worksheet based on the graph of
Stewart’s trip.
300
250
200
150
100
50
0
0
20
40
Tim e (hours)
13
Distance V. Time Graphs
 A passenger train leaves station C and






© 2001-2007 Shannon W. Helzer. All Rights Reserved.
Distance v. Tim e
30
25
20
Distance (km)

travels East to station B.
It then travels West to station A before
returning to station C.
Answer the following questions using the
graph provided to the right.
What was the trains velocity between 0 and
20 minutes, at 1 hour, and 2 hours 36
minutes into the trip?
How far from station B is Station C?
How far from station A is Station B?
What was the train doing at the circled
point on the graph?
Is this trip a round trip? Why or why not?
15
10
0
5
0
-5 0
1
2
3
4
5
-10
Tim e (hr)
Rise d d 2  d1
v


Run t
t 2  t1
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Velocity v. Time
 The ship below is initially at rest.
 Notice what happens when a positive
Rise v v2  v1
a


Run t t 2  t1
© 2001-2007 Shannon W. Helzer. All Rights Reserved.
10
Velocity (m/min)
acceleration acts upon the ship.
 What do you suppose would happen if a
negative acceleration acted on the ship?
 Calculate the acceleration acting on the
ship.
Velocity v. Time
5
0
0
5
10
15
20
-5
Time (min)
15
Velocity v. Time Graphs (WS 6 1-7)
 The graph to the right depicts a short trip
Rise v v2  v1
a


Run t t 2  t1
© 2001-2007 Shannon W. Helzer. All Rights Reserved.
5
Velocity (m/s)
taken by MeanyBot.
 Answer the questions on your work sheet
using the graph provided.
 Please note that the slope of a V v. T
graph is the acceleration of the moving
body.
Velocity v. Time
4
3
2
1
0
0
4
8
12
Time (s)
16
Velocity v. Time Graphs
 On this slide we will explore the effects of
Rise v v2  v1
a


Run t t 2  t1
© 2001-2007 Shannon W. Helzer. All Rights Reserved.
7.5
Velocity (m/min)
a negative velocity on the direction of
travel.
 What is the velocity between 4 and 6
minutes, at 8 minutes, and at 14 minutes.
 What is the acceleration between 4 and 6
minutes, at 8 minutes, and at 14 minutes.
 Physically what is MeanyBot doing at the
point indicated on the graph?
Velocity v. Time
2.5
0
10
20
-2.5
-7.5
Time (min)
17
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