# Dr. Joseph W. Howard ©Spring 2008 “Speed” is mostly a description of how fast an object is going “Speed” i s mostly a description of how fast an object.

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Dr. Joseph W. Howard ©Spring 2008 “Speed” is mostly a description of how fast an object is going “Speed” i s mostly a description of how fast an object is going

Dr. Joseph W. Howard ©Spring 2008 Speed SPEEDWhen discussing the motion of an object, we also need to refer to an object’s “rate at which it’s position changes” – SPEED quickly How quickly did it move ? slowly How slowly is it moving ? fast How fast was the motion ? distance time Speed =

Dr. Joseph W. Howard ©Spring 2008 Careful about Speed Average Speed Instantaneous Speed ????

Dr. Joseph W. Howard ©Spring 2008 Average Speed: Total Distance Traveled Total Time Spent Traveling Instantaneous Speed: Your Speed at any Given Instant

Dr. Joseph W. Howard ©Spring 2008 Formulae distance time Speed = (Average Speed)  (Elapsed Time) Total Distance Traveled = Average Speed = Total Distance Traveled Elapsed time

Dr. Joseph W. Howard ©Spring 2008 Example Problems A 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? 0.0 meters!! Why?

Dr. Joseph W. Howard ©Spring 2008 Example Problem 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? 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?

Dr. Joseph W. Howard ©Spring 2008 Example Problem 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?

Dr. Joseph W. Howard ©Spring 2008 Example Problem 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?

Dr. Joseph W. Howard ©Spring 2008 Conceptual Pitfall Example 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. W hat is their average speed?

Dr. Joseph W. Howard ©Spring 2008 Relative Speed 60 mph 130 mph Relative to each other, their speeds are 130 – 6 0 mph = 7 0 mph Conceptual Pitfall

Dr. Joseph W. Howard ©Spring 2008 Speed & Velocity

Dr. Joseph W. Howard ©Spring 2008 Difference 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 Velocity Describes how an object ’ s displacement changes over time. Describes how an object ’ s displacement changes over time.

Dr. Joseph W. Howard ©Spring 2008 Average Speed  0 Average Velocity = 0 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 Average Velocity

Dr. Joseph W. Howard ©Spring 2008 When Speed is Constant, but Velocity is not! Conceptual Pitfall

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