Speed & Velocity
How fast How far is changing.
The rate at which an object moves.
distance
Velocity
Velocity is a vector. It has both magnitude (speed) and direction.
Instantaneous velocity is the velocity at any instant.
distance
Speed & Velocity
total distance traveled Δd
Average Speed =
=
Δt
total time
m
[speed ] = ,
s
[distance] = m
•Speed: Scalar (magnitude)
•Velocity: Vector (magnitude AND direction)
•Unit is m/s
• s and v often interchanged …..
Speed Question
A motorcycle travels 1000 m in 2 minutes.
What is its average speed?
total distance traveled
Average Speed =
total time
Distance
total distance traveled
Average Speed =
total time
Average Speed x total time= total distance traveled
d = vaveraget
If you travel an average of 80
km/hr for 15 minutes, how far have
you traveled in that time?
Acceleration
How fast How fast is changing.
The rate at which the speed is changing.
change in velocity Δv
=
a=
change in time
Δt
Speeding up
Slowing down
Speeding up or slowing down?
Velocity from Acceleration
change in velocity Δv
=
a=
change in time
Δt
Δv = aΔt
v = at
If you start from rest and accelerate at
a rate of 5m/s every second (5 m/s2)
for 10 seconds, what will be your
final speed?
Acceleration and velocity do not
have to be in the same direction.
Motion Question
Can an object move if the net forces acting on it are zero?
Give an example.
Can an object be stationary but still be accelerating?
Give an example.
Hill Question
a)
b)
c)
On which of these hills does the ball roll down with increasing
speed and decreasing acceleration? a)
b)
c)
Tracks Question
Two balls are released simultaneously from rest at the left
end of equal length tracks A and B as shown. Which ball
reaches the end of its track first?
Free Fall
In the absence of air resistance,
all objects fall due to the force of
gravity with an acceleration “g”:
g = 9.8m / s ~ 10m / s
2
2
Free Fall
In a vacuum, all objects fall with the same
acceleration due to gravity: 10 m/s2.
Free Fall Speed
a = g = 10m / s
v = at
v = 10t
d = 5t
2
Do Work Book
2
Free Fall: Throwing UP
What is the speed at the top of the
path?(assume g = 10m/s2 )
ZERO!
What is the acceleration at the top?
g=10m/s2
What is the speed at the same
height on the way down?
-30 m/s
+y
Free Fall Symmetry
Which ball has the greatest speed when it hits the ground?
SAME!
The gravitational field is symmetric
(in the absence of air resistance.)
Free Fall Summary
For an object without an initial velocity and without air resistance.
a = g = 10m / s
v = 10t
2
d = 5t
2
If there is an initial velocity, then add it!
v(t ) = vi + 10t
d (t ) = vi t + 5t
Do Aunt Minnie problems in the Practice Book.
2