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 Chapter 2
Sections 1-3
Section 1 - Describing Motion slides 3- 21
Section 2 - Acceleration slides 22 – 28
Section 3 – Motion & Forces slides 29 -41
Section 1- Describing Motion
What You’ll Learn:
The difference between displacement &
The difference between speed & velocity
How to graph motion
Distance & time are important in describing a
race. The winner covers the distance in the
shortest amount of time.

How are motion & position related?
 A reference point is needed to tell where
 something is or if it has moved.

   What is relative motion?
Not all motion is obvious. When you are
sitting still, you appear not to be moving
although you are moving in relation to the
Sun because you are sitting on Earth.
Relative motion means that one thing
moves in relation to another thing. Earth
is moving in space in relation to the Sun.
The Sun is the reference point for the
Earth’s motion.
How are distance & displacement
Distance is how far
something has moved.
The distance of a 50 m
dash is 50 meters.
Displacement is the
distance & direction of
an object’s position
relative to the starting
point.                     Running of the Bulls
                           (Pamplona, Spain)
To tell how fast something is moving, you need
to know how far it travels in a certain amount of
Speed is the distance an object travels per unit
of time.
                       “Hundreds of people and six fighting
                       bulls, accompanied by steer, sprinted
                       the half-mile (850-meter) route through
                       cobblestone streets in just over two
                       minutes…” Associated Press 7-8-2008
                       Speed = 850m / 2 min= _______ m/s
   How is speed calculated?
Speed = distance/time= m/s
850m/ 2 min or 120 sec= 7.08 m/s
We divide the distance traveled by the time it
took to travel the distance.

                       “Hundreds of people and six fighting
                       bulls, accompanied by steer, sprinted
                       the half-mile (850-meter) route through
                       cobblestone streets in just over two
                       minutes…” Associated Press 7-8-2008
                       Speed = 850m / 2 min= _______ m/s
          What is motion with constant
      A speedometer measures the speed of a
      car. Suppose you look at it when you are
      riding on a freeway. The car is not
      speeding up or slowing down; it is moving
      at a constant speed.

   What is changing speed?
Usually speed is not      Speed Changing Over Distance
                          25   Going downhill
Here we see how the
                     S                           Stopping for
speed of a cyclist   p
                          20                     red light
changes over 5 km. e           steep hill
                     e    15                    Constant speed
The ride took 15     d
minutes.             (k   10
                     m         Gaining speed


   What is average speed?
Average speed is the total distance
traveled divided by the total time of travel.
It is calculated using the relationships
among speed, distance & time.
Average speed= total distance / total time=
5 km/15 min or .25 hr= 20 km/hr.
What is instantaneous speed?
Instantaneous speed is the speed at one
point in time, or one instant.
If an object moves with constant speed,
the speed remains the same; however, if it
speeds up or slows down, the
instantaneous speed is changing.
             Graphing Motion
shows the
motion of
an object
over time.
          Graphing Motion
The steepness of the
line is its slope (m).
The slope of the line
is the speed. A
steeper slope means
a greater speed.
Slope= rise/run or

 How is changing speed graphed?
                                        The changing slope in
                                        this graph indicates
                                        changing speed.
                                        Review graphing from
                                        Chapter 1 notes.

How do you draw a distance-time
Distance is plotted on the vertical (y) axis
& time is plotted on the horizontal (x) axis.
Each axis has a scale, or series of
numbers, that covers the range of the
Each axis is divided into equal parts; then
the data points are plotted on the graph
and finally connected with a line.
Suppose there is a storm nearby traveling
at a speed of 20 km/h that is 20 km east of
your location. Will it reach you?

       <Associated Press, 2008>
Suppose there is a storm nearby traveling
at a speed of 20 km/h that is 20 km east of
your location. Will it reach you?
Knowing the speed is not enough; you
must also know the direction it is traveling.
Velocity is the speed of an object and the
direction it is moving.
How do speed & direction affect
Two escalators may move at the same
speed even though one goes up and the
other travels down.
Because they move in opposite directions
their velocity is different despite having the
same speed.
Velocity depends on both speed and

     Motions of Earth’s Crust
Some motion is so slow that it is hard to
see such as plate tectonics. Geologic
evidence shows that the continents have
moved over millions of years.
The plates of the lithosphere slide on the
softer layers below manifesting
themselves as earthquakes, volcanic
eruptions and formation of mountains.
The plates along San Andreas Fault speed along at about 1 cm/yr!
    Section 2 - Acceleration
What You’ll Learn:
How acceleration, time & velocity are
How positive & negative acceleration
affect motion
How to calculate acceleration
Acceleration, Speed, & Velocity
Acceleration is the rate of change of
velocity. When velocity changes, the
object is accelerating.
Speed is the rate of change of position.
Remember velocity includes both speed &
How are speeding up & slowing
      down described?
Speeding up & slowing
down are both
The first is positive and
the second is negative.
Acceleration has
direction, just like

Does changing direction affect
                        The speed of
                        a horse
                        around a
                        constant, but
                        because it is
                        direction it is
     Calculating Acceleration
Find the change in velocity by subtraction.
∆V= final V – initial V or Vf – Vi
Then, to find acceleration, divide ∆V by the
length of time during the velocity change.
Acceleration (α)= ∆v or Vf – Vi /time (t)

where ∆ means change, V is velocity, α is acceleration, etc.
    Calculating Acceleration
The SI unit for velocity is meters/sec (m/s).
To find acceleration, velocity is divided by time in
seconds (s).
The unit for acceleration is m/s2.

Amusement Park Acceleration
Roller coasters are exciting rides because
their designers use the laws of physics.
The steep drops and loops of steel roller
coasters give the rider large accelerations
by using gravity and sharp turns.

Section 3 – Motion & Forces
What You’ll Learn:
How force & motion are related
Newton’s first law of motion
What forces & motion are present during a
car crash
             What is force?
Force is a push or a
pull like kicking a
soccer ball.
Sometimes it isn’t so
obvious. Can you
feel gravity holding
you in place?
You exert force when
you sit, stand, stretch,
bend, push or pull.

How does force change motion?
                           What happens to the
                           motion of the ball
                           when the racket hits

How does force change motion?
                           What happens to the
                           motion of the ball
                           when the racket hits
                           The force of the
                           racket makes the ball
                           stop and then move in
                           a different direction.

  What are balanced forces?
When 2 or more
forces act on an
object at the same
time, the combined
force is called net
force. Forces that
are equal in size &
opposite in direction
are called balanced
forces.                 <>
What is the result of unbalanced
                 If both students push
                 equally in opposite
                 directions, the book
                 does not move.
                 If the student on the
                 right pushes harder as
                 shown, movement will
                 be to the left.
                 Net force is the sum of
                 all forces acting on the
            Inertia and Mass
Inertia is the tendency of an object to resist any
change in its motion.
If it’s moving, it will keep moving until a force
changes either its direction or speed.
A dirt bike will move in a straight line with a
constant speed until a force acts on it. A force
can turn the wheel to change direction or friction
can slow the speed.
If it’s motionless, it still has inertia & will remain
motionless until an unbalanced force causes it to
 Why can different objects have
       different inertia?
Inertia is related to
the mass of the
Mass is the amount of
matter in an object.                      .com>

A bowling ball has
greater inertia than a
table tennis ball.

What are Newton’s laws of motion?

 British scientist, Sir Isaac Newton, stated rules
 that describe the effects of forces on the motion
 of objects.

 They apply to all objects in motion from billiard
 balls to planets.
 What is Newton’s First Law of
An object moving at a constant velocity
keeps moving at that velocity unless an
unbalanced force acts on it.
An object at rest remains at rest unless a
net force acts on it.
Newton’s first law of motion is sometimes
called the law of inertia.

     What happens in a crash?
The law of inertia
explains what
happens in a car
crash. Passengers
who are not wearing       <>

a safety belt continue
to move at the
original speed of the
car despite the car
coming to slamming
halt.                    <>
  How do safety belts help?
A person wearing a safety belt is attached
to the car so they will slow down as the car
slows down.
The safety belt prevents the passenger
from slamming into the dashboard,
windshield, or seat backs.
It also helps prevent people from being
thrown from the car. Half the people who
die in crashes would survive if they wore
safety belts.
   In a 30 m.p.h. collision an …
In a 30 m.p.h. collision an unbelted 160 lb. Person can strike another
passenger, crash through a windshield and/or slam into the vehicle’s
interior with a 4,800 lb. force.


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