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Dr. Joseph W. Howard
©Spring 2008
“Speed” is mostly a description of how fast an object is going
“Speed” is mostly a description of how fast an object is going
Dr. Joseph W. Howard
©Spring 2008
SpeedSpeed• When discussing the motion of an object, we
also need to refer to an object’s “rate at which it’s position changes” – SPEEDSPEED
• How quicklyquickly did it move?• How slowlyslowly is it moving?• How fastfast was the motion?
distance
timeSpeed =
Dr. Joseph W. Howard
©Spring 2008
Careful about SpeedCareful about Speed
Average SpeedAverage Speed
Instantaneous SpeedInstantaneous Speed
????
Dr. Joseph W. Howard
©Spring 2008
Average Speed:Average Speed:
Total Distance TraveledTotal Distance Traveled
Total Time Spent TravelingTotal Time Spent Traveling
Instantaneous Speed:Instantaneous Speed:
Your Speed at any Given InstantYour Speed at any Given Instant
Dr. Joseph W. Howard
©Spring 2008
FormulaeFormulae
distance
timeSpeed =
(Average Speed) (Elapsed Time)Total Distance Traveled =
Average Speed =Total Distance Traveled
Elapsed time
Dr. Joseph W. Howard
©Spring 2008
Example ProblemsExample ProblemsA dog runs at full speed of 3.0 m/s. How many meters in distance does the dog run in 2 minutes?
A dog runs at full speed of 3.0 m/s. How many meters in distance does the dog run in 2 minutes?
If the dog runs half the time one direction then reverses and runs the other minute, what is the dogs displacement?
If the dog runs half the time one direction then reverses and runs the other minute, what is the dogs displacement?
2 minutes 60 seconds
1 minute
= 120 seconds
mDistance = 3.0 × 120s
s
distance traveledAvg. Speed =
time elapsed time time
= 360meters
0.0 meters!! Why?0.0 meters!! Why?
Dr. Joseph W. Howard
©Spring 2008
Example ProblemExample ProblemA rocket car trying to break the land speed record of 600 mph (average speed) manages to drive a distance of 604 meters in 2.12 seconds. Does the rocket car break the record?
A rocket car trying to break the land speed record of 600 mph (average speed) manages to drive a distance of 604 meters in 2.12 seconds. Does the rocket car break the record?
distance traveledAverage Speed =
elapsed time
distance traveledAverage Speed =
elap
6
se
04
d ti
m=
2m .12se
distance traveled 604mAverage Speed = =
elapsed ti
m=
me 2.1 s285
2 s
Dr. Joseph W. Howard
©Spring 2008
distance traveled 604m mAverage Speed = = = 285
elapsed time 2.12s s
Example ProblemExample ProblemA rocket car trying to break the land speed record of 600 mph (average speed) manages to drive a distance of 604 meters in 2.12 seconds. Does the rocket car break the record?
A rocket car trying to break the land speed record of 600 mph (average speed) manages to drive a distance of 604 meters in 2.12 seconds. Does the rocket car break the record?
m285
s
60s
1min
m285
s
Dr. Joseph W. Howard
©Spring 2008
m285
s
60 s
1min
distance traveled 604m mAverage Speed = = = 285
elapsed time 2.12s s
Example ProblemExample ProblemA rocket car trying to break the land speed record of 600 mph (average speed) manages to drive a distance of 604 meters in 2.12 seconds. Does the rocket car break the record?
A rocket car trying to break the land speed record of 600 mph (average speed) manages to drive a distance of 604 meters in 2.12 seconds. Does the rocket car break the record?
285×60×60 miles
1609 hour
60min
1hour
1mile
1609m
mileshour = 638
Dr. Joseph W. Howard
©Spring 2008
Conceptual PitfallConceptual PitfallExample ProblemExample Problem
(Average Speed) (Elapsed Time)Total Distance Traveled =
Jack and Jill walk 12 meters up a hill at a speed of 2 m/s and walk down the hill at a speed of 4 m/s. What is their average speed?
12 meters meters
2 second
time up the hill
time up the hill 6 seconds
12 meters time down the hill
time down the hill 3 seconds
meters4
second
total distanceAverage Speed =
elapsed timem
= 2.67 s
24 meters=
9 seconds
Dr. Joseph W. Howard
©Spring 2008
Relative SpeedRelative Speed
60 mph
130 mph
Relative to each other, their speeds are 130 – 60 mph = 70 mph
Conceptual PitfallConceptual Pitfall
Dr. Joseph W. Howard
©Spring 2008
Speed & VelocitySpeed & Velocity
Dr. Joseph W. Howard
©Spring 2008
Difference Between Speed & VelocityDifference Between Speed & Velocity
Same Lengths Same Speeds
Different Directions Different Velocities
Same Lengths Same Speeds
Different Directions Different Velocities
Dr. Joseph W. Howard
©Spring 2008
Average VelocityAverage Velocity
Describes how an object’s displacement changes over
time.
Describes how an object’s displacement changes over
time.Displacement
Average Velocity = elapsed time
Dr. Joseph W. Howard
©Spring 2008
Average Speed 0Average Velocity = 0
Turn around and ride to the North, covering 6 miles also in 2 hours
Turn around and ride to the North, covering 6 miles also in 2 hours
Ride a bicycle to the South, covering 6 miles in 2 hours
Ride a bicycle to the South, covering 6 miles in 2 hours
Average VelocityAverage Velocity
Dr. Joseph W. Howard
©Spring 2008
When Speed is
Constant, but
Velocity is not!
When Speed is
Constant, but
Velocity is not!
Conceptual PitfallConceptual Pitfall
