What is speed?
Speed: how fast something is moving or how much
distance is covered in a certain amount of time.
We will discuss two types of speed:
• Instantaneous speed: An object’s speed at any
given instant.
• Average speed: An average of all instantaneous
speeds.
To calculate average speed
• average speed = total distance/time
s = d/t
If you cover 75 m in 3 sec, how fast are you going?
Answer: 25 m/s
If you travel 20 m/s for 10 m, how long did it take
you?
Answer: .5 s
Velocity
• Velocity is the same as speed, but it has a
direction associated.
• Speed has no specific direction.
Question
• Can something have a constant speed but a
changing velocity?
• Yes. All it has to do is turn.
• Remember: Velocity includes direction;
speed does not.
Negative velocity and speed
Is it possible for something to have a negative
velocity?
• Yes, it means it is going backward.
Is it possible for something to have a negative
speed?
• No, there is no specific direction. Backward is
arbitrary. You pick which way you want to be
positive in any given problem.
Combining vectors in 1-D
• First, simply add them together.
• If you walk 5 miles north, 2 miles south and 3
miles north, what distance have you walked?
What is your displacement?
– Distance: 10 m
– Displacement: 6 m N
• If you are walking toward the back at
1 m/s on a bus that is moving 10 m/s …
– You are moving 9 m/s (forward)
Note: You cannot combine speeds.
Vectors
• Vectors are units with a direction associated with
them.
– Distance: no direction
– Displacement: distance with a direction
• If you walk 5 m north and 2 m south, what distance
have you walked? What is your displacement?
• Distance 7 m; displacement 3 m N