Transcript Chapter 2- Motion and Energy

Chapter 2
Chapter 2.1

Objectives
 Compare frames of reference.
 Distinguish between speed and velocity.
 Calculate when a moving object will arrive at a
given point.
 Make a graph to solve a distance-time problem.
Imagine you are
traveling across the
ocean on a ship. You
see another ship
getting closer.
 Are both ships
moving or is only the
ship you are on
moving?
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It isn't always easy to
decide when objects
are moving.
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You judge motion in
relation to stationary
objects. This is called
a frame of reference.
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A frame of
reference is a
place or object
that you assume is
fixed.
You observe how
objects move in
relation to that
frame of
reference.
What does this
photographs tell
you about the
motion of the
car?
You use the earth’s surface as your frame of reference most of the
time. However, you experience moving frames of reference when
you ride in a vehicle, such as a car, a bus, or an elevator.
Two persons with the same
frame of reference see the
motion of an object the
same way. Both boys on
the bus see the ball drop
straight down.
Two persons with different frames
of reference see an object’s motion
differently. Both boys on the bus
see the ball drop straight down. To
a bystander, the ball moves in two
directions—forward with the bus &
down toward its floor.
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http://www.youtube.com/watch?v=7FYBG5G
SklU
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Speed = Distance/Time
Speed is the
distance an object
travels in a certain
amount of time.
To calculate speed,
you divide the
If the cyclist rode his bike 8 km to school
distance traveled
in 20 minutes, you can calculate his speed
by the time it took
in km/h as follows:
to travel that
Speed : 8 km/20 min x 60 min/1 h
distance.
= 24 km/h
A weather vane indicates wind
direction. If it blows at 40 km/h
what is its velocity?
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The speed and the
direction of an object's
motion are called
velocity.
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Since a moving object
always travels in some
direction, velocity is a
more accurate term
for describing motion.
A moving object that doesn't change its speed travels
at constant speed.
 Constant speed means equal distances are covered in
an equal amount or time.
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A
distance-time
graph of constant
speed forms a
straight line.
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A distance-time graph of a marathon shows that
the runner's speed changed several times.
To find the speed of the runner during
the entire race, you need to calculate
his average speed.
Average speed is equal to the total
distance traveled divided by the total
time.
What was the average speed
of the marathon runner?
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1. A kayak races 100 m in 50 s. What is its speed?
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Speed = distance / time
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100 m/ 50 s = 2 m/s
Now do 3 & 5
3. Distance=3m/s x 35s = 105 m
Distance = 3m/s x 60s/min x 60 min = 10,800 m
5. 7.5 km / 5h = 1.5 km/h
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We know observations of motion depend on
your frame of reference.
Yet, experiments show the speed of light is
always the same, regardless of the motion of
the light source or the motion of the observer!
To understand this, think about a rocket docked
on the earth and another rocket traveling
directly toward the sun at great speed…
The light from the sun reaches both rockets at
the same speed (300 million meters/s).
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The constant speed of light is the basis for
Albert Einstein's special theory of relativity.
He reasoned that space and time are connected
into one whole, called space-time.
You are constantly traveling through this
combination of space and time.
When you stand still, you travel only through
time.
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When you move, you travel through space and
time.
If you could move at close to the speed of light,
you would travel quickly through space and
slowly through time.
http://www.youtube.com/watch?v=KYWM2oZgi
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Answer questions 1 & 2. Use complete
sentences.
1. A frame of reference is usually fixed and is
used to describe the motion of objects relative
to it.
2. the velocity of the bird include the direction
the bird is moving.
Chapter 2.2

Objectives
 Define operationally the acceleration of an
object.
 Relate motion in a circle to acceleration.
 Contrast acceleration and constant speed.
 Make a graph showing acceleration.
 Any change in velocity is called an
acceleration.
 Acceleration can be speeding up,
slowing down, or changing the
direction of the motion.
 All of these accelerations, or velocity
changes, require an outside force.
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Imagine you are competing as a speed skater at the
Winter Olympics.
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During practice, you learned that three changes in
speed were the key to winning the race:
 speed up whenever possible
 slow down with control when necessary
 and change direction as smoothly
as possible.
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When you mastered these three
things (changes in velocity), you
are ready to compete.
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The rate at which velocity changes occur is called
acceleration.
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To calculate acceleration you divide the change in
velocity by the amount of time.
Acceleration = Change in velocity (final velocity – starting velocity)
time
Suppose you ride a bike on a straight path to school
at a velocity of 4 m/s. As you get closer, you hear the
school bell. In 3 s, you speed up to 10 m/s. How
would you calculate your acceleration?
Change in Velocity = final velocity - starting velocity
10 m/ s – 4 m/ s = 6 m /s
Acceleration =
A Graph of Positive Acceleration
To think about positive
acceleration, imagine a car
waiting at a stoplight. When
the light turns from red to
green, the driver steps on the
accelerator, and the car
speeds up. As the car moves
faster, you feel the change in
motion as your body is
pushed back against the seat.
How many seconds did the car
travel at a constant positive
acceleration?
A Graph of Negative Acceleration
To think about
negative acceleration,
imagine a car slowing
down. The car's
velocity decreases over
a certain amount of
time. This type of
velocity change is also
called deceleration.
During what 3 second interval did
the car decelerate fastest?
Each change in direction
represents a change in
velocity.
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Also, whenever the
direction of a moving
object changes, the
velocity of the object
changes.
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Remember, any change
in velocity is
acceleration—even if
the speed of the object
remains the same.
A Racer's Acceleration
Each change in direction represents a change in velocity.
Suppose you are a race car driver. At the beginning of the
race, all of the cars start from rest and accelerate straight
down the race track before reaching the first corner.
Your car's speed after the first few seconds is given in the
table.
Use the data in the table to plot a speed-versus-time
line.
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Answers:
No. Constant acceleration would produce a
linear graph.
Final velocity, starting velocity and time.
8 m/s2 , 8 m/s2, 6 m/s2, and 4 m/s2 respectively.
Greatest: 0-2 sec., Least: 3-4 sec.
26 m/s – 0 m/s = 6.5 m/s2
4s
The friend will be in the lead.
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An object moving in a
circle or a curve is
constantly changing
direction, therefore, the
object is accelerating.
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Acceleration caused by
motion in a circle is called
centripetal acceleration.
What would happen to the
eraser if the girl let go of
the string? Why?
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Objectives
 Define energy.
 Explain the law of conservation of energy.
 Compare and contrast potential energy
and kinetic energy.
 Infer the gravitational potential energy of
everyday objects.
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Changes in motion occur constantly in
the world around you.
 Cars move on busy highways.
 Tons of rock hurtle down mountainsides.
 Underground water is brought to the surface for crop
irrigation.
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What gives the cars, rocks, and water
the ability to move?
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Any change in motion
requires energy.
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Energy is the ability to
do work.
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When work is done, a
change occurs.
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One might say that
energy is the source of
change.
The archer uses energy to pull
the bow's string toward her to
change the string's position.
When she lets go, energy in the
string transfers to the arrow,
and the arrow moves forward.
Stored energy
has the ability to
do work and is
called potential
energy.
 Potential energy
is associated
with position.
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Energy of motion is called kinetic energy.
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The amount of kinetic energy depends on the
moving object's mass and velocity.
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To calculate kinetic energy, you multiply one half
of the object's mass times the square of its
velocity.
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If the mass of a bowling ball is 4 kg and its
velocity is 5 m/s, how much kinetic energy does it
have?
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Energy cannot be created or destroyed.
This is known as the law of conservation of
energy.
Energy can change into other forms, but the
total amount of energy never changes.
You may infer that a rock on
top of a cliff has potential
energy because of its position.
Once over the cliff’s edge, the
force of gravity pulls the rock
downward. The rock's energy is
due to both its position on the
cliff and the force of gravity.
This type of energy is called
gravitational potential energy.
Turn to page 49 in your text
book.
Read “SCIENCE &
TECHNOLOGY” Great Potential
Look at figure 2.19
Answer the question:
How is a water wheel like a
dam?