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Bell Work
distance
speed 
time
 If
you ran 10 meters in 5 seconds,
what was your average speed?
 If
you ran at an average speed of 4
m/s and you ran for 10 seconds,
how far did you travel?
Ch. 3
Motion, Acceleration &
Forces
I.
III.
Describing Motion
II.
Acceleration
Motion and Forces
A. Motion
 Problem:
 Is your desk moving?
 We
need a reference point...
 nonmoving point from which
motion is measured
A. Motion
 Motion
 Change in position in relation to
a reference point.
Reference point
Motion
A. Motion
 Motion
is Relative
 Examples:
• Traveling on a moving
walkway at the airport
• Person walking on a boat or
a train
• Boat traveling along a flowing
stream
A. Motion
Problem:
 You are a passenger in a car
stopped at a stop sign. Out of the
corner of your eye, you notice a
tree on the side of the road begin
to move forward.
 You have mistakenly set yourself
as the reference point.
A. Motion
 Distance
and Displacement
Distance is the
length of the path
traveled
Displacement is
the distance and
direction of an
object’s final
position from its
initial position.
A quantity that includes size and direction is a vector
Distance and Displacement
B. Speed
 Speed
 rate of motion
 distance traveled per unit time
 SI units: meters/second (m/s)
distance
speed 
time
B. Speed
 Instantaneous
Speed
 speed at a given instant
 i.e. a car’s speedometer
B. Speed
 Average
Speed
total distance
avg. speed 
total time
Calculations
 If
Speed = Distance/Time
 Then Time = ?
Distance/Speed
 Then Distance = ? Speed X Time
Practice

A football field is about 100 m long. If it takes
a person 20 seconds to run its length, how
fast (what speed) were they running?
Given:
D= 100 m
T = 20 s
S=?
Equation:
S = D/t
S = 100/20 = 5 m/s
Practice

The pitcher’s mound in baseball is 85 m from
the plate. It takes 4 seconds for a pitch to
reach the plate. How fast is the pitch?
Given:
D= 85m
T = 4s
S=?
Equation:
S = D/t
S = 85/4 = 21.25 m/s
Practice

If you drive at 100 km/hr for 6 hours, how far
will you go?
Given:
D= ?
T=6h
S = 100 km/hr
Equation:
D=SxT
D = 100 x 6 = 600 km
Practice

If you run a t 12 m/s for 15 minutes, how far
will you go?
Given:
D= ?
T = 15 min OR 900 s Equation:
S = 12 m/s
D=SxT
D = 12 (m/s) x 900s =
10,800 m
Practice

The fastest train in the world moves at 500
km/hr. How far will it go in 3 hours?
Given:
D= ?
T = 3 hr
S = 500 km/hr
Equation:
D=SxT
D = 500 x 3 = 1500 km
Practice

A bullet travels at 850 m/s. How long will it
take a bullet to go 1 km?
Given:
D= 1 km OR 1000 m
T=?
Equation:
S = 850 m/s
T = D/S
T = 1000/850 = 1.18 s
Bell Work
 If
you drive at 100 km/hr for 6
hours, how far will you go?
A
car’s speedometer is an
example of what?
B. Speed & Velocity
 Problem:
 A storm is 10 km away and is
moving at a speed of 60 km/h.
Should you be worried?
 It depends
on the
storm’s
direction!
B. Speed & Velocity
 Velocity
 speed in a given direction
 can change even when the
speed is constant!
Graphing Motion
Graphing Motion
Graphing Motion –
summary
In which graph are the runners
moving at the same speed?
Match the description to
the graph
Bell Work – 1st/7th
Bell Work – 3rd
C. Acceleration
vf - vi
a t
 Acceleration
 the rate of change of velocity
 change in speed or direction
a
v f  vi
t
a:
vf:
vi:
t:
acceleration
final velocity
initial velocity
time
Which graph shows
acceleration?
400
Distance (m)
300
200
100
0
0
5
10
Time (s)
15
20
C. Acceleration
 Positive
acceleration
 “speeding up”
 Negative
acceleration
 “slowing down”
Graphing Acceleration
Speed-Time Graph
Specify the time period
when the object was...
 slowing down
 5 to 10 seconds
 speeding up
 0 to 3 seconds
3
Speed (m/s)
2

1
0
0
2
4
6
Time (s)
8
10

moving at a constant
speed
 3 to 5 seconds
not moving
 0 & 10 seconds
Match the description
Calculations
A roller coaster starts down a hill at 10 m/s.
Three seconds later, its speed is 32 m/s.
What is the roller coaster’s acceleration?
GIVEN:
WORK:

vi = 10 m/s
t=3s
vf = 32 m/s
vf - vi
a=?
a t
a = (vf - vi) ÷ t
a = (32m/s - 10m/s) ÷ (3s)
a = 22 m/s ÷ 3 s
a = 7.3 m/s2
Acceleration Calculations
INITIAL VELOCITY
•70 mps
•40 mps
•9.8 mps
•9.8 MPH
FINAL VELOCITY
2 mps
100 mps
129 mps
120 MPH
TIME
2 seconds
10 seconds
5.5 seconds
1 hour
Practice

A roller coaster car rapidly picks up speed as it rolls
down a slope. As it starts down the slope, its speed
is 4 m/s. But 3 seconds later, at the bottom of the
slope, its speed is 22 m/s. What is its average
acceleration?

A cyclist accelerates from 0 to 8 m/s in 3 seconds.
What is his acceleration? Is this acceleration higher
than that of a car which accelerates from 0-30 m/s in
8 seconds?
Bell Work
 What
is the acceleration of a car
that travels in a straight line at a
constant speed?
 Describe
a situation in which you
can accelerate even though your
speed doesn’t change.
Worksheet/Quiz Review
Worksheet/Quiz Review
Chapter 3 Section 3
Motion and Forces
Motion and Forces
A. Force

Force
 a push or pull that one object exerts
on another
 In SI units, force is measured
in newtons (N)
Fkick
Fgrav
A. Force

Net Force
 Two or more forces act on an object at the
same time, the forces combine
 Forces can be balanced or unbalanced.
A. Force

Balanced Forces
 forces acting on
an object that
are opposite in
direction and
equal in size
 Net force = zero
 Velocity =
unchanged
A. Force

Unbalanced Force
 Forces combine to produce a net
force not equal to zero.
 velocity changes (object accelerates)
Fnet
Ffriction
Fpull
N
N
W
Unbalanced forces change
velocity
B. Friction

Friction
 force that opposes sliding motion between
2 surfaces in contact
 depends on the:
• types of surfaces
• force between them
surfaces
When is friction
greater?
Comparing Friction Demo



Ice cube, rock, eraser, wood block and
square of aluminum foil at one end of a metal
tray.
Predict the order that they will begin to slide
as you raise the tray.
How did the height that sliding began depend
on roughness? On weight (size of force)
Bell Work
 Friction
depends on two things.
What are they?
Types of friction
Static Friction: prevents surfaces in
contact from sliding past each other
 Sliding friction: a force opposing the
sliding motion.

C. Air Resistance
 What
will fall faster? The paper
clip or the rock?
 A feather or the rock?
C. Air Resistance

Air Resistance
 force that air exerts
on a moving object
to oppose its motion
 depends on:
• speed
• surface area
• shape
• density of fluid
C. Air Resistance

Terminal Velocity
 increasing speed  increasing air
resistance until…
Fair = Fgrav
Animation from “Multimedia Physics Studios.”
C. Air Resistance

Terminal Velocity
 maximum velocity reached by a
 no net force
falling object
 no acceleration
 reached when…
 constant velocity
Fgrav = Fair
Bell Work
1.
2.
3.
4.
Convert 80 cm to meters.
Convert 565 mm to meters.
Convert 5 km to meters.
Convert 40 km/h to m/s
Bell Work
Convert 80 cm to meters.
100 cm in a meter. Move the decimal 2 to left =
0.8 m
Convert 565 mm to meters.
1000 mm in a meter. Move the decimal 3 to the
left. = 0.565 m
Convert 5 km to meters.
1000 meters in a Km. Move the decimal 3 to the
RIGHT. 5000 m
Convert 40 km/h to m/s
40 km/hr x 1000 m/km x 1 hr/3600s = 11.11 m/s2
Cram.com flashcards
 www.cram.com/flashcards/motion
-acceleration-and-forces-5406469
 www.cram.com/flashcards/motion
-acceleration-and-forces-level-25434275
Additional activity
 Rocket
 LINK
Sledder