Download Chapter 13 Motion

Chapter 13
Motion
DO NOW
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What kinds of things have motion?
What do you know about motion??
How can you describe motion?
What are some motions you went through
today?
• Sitting at your desk, are you moving in relation
to your chair?
• Are you moving in relation to anything???
Types of Motion
It’s all around you.
Different motions happen at different rates
It can be steady (constant) or variable (changing).
It can be in many directions.
Periodic (pattern) /circular/vibration
Rate
• When do you see motion at different rates?
When do you see constant motion?
When do you see variable motion?
Motion in many directions
Periodic Motion
Circular Motion
Vibrating Motion
DO NOW:
Set your pen or pencil in front of you on
your desk.
Use your finger to pull the pencil toward
you.
Use your finger to push the pencil away
from you.
What are you using to move the pencil?
(besides your finger)
DO NOW
• What is a force?
• When have you heard the word “force” used
before?
• What are some qualities of forces? (ways you
could describe forces)
What
are
forces?
PUSHES AND PULLS!
• Force = push or pull that acts on an
object.
• One object may exert a force on
another object
What do you
know about
the word
“magnitude”
?
Characteristics of Forces
1. Magnitude
2. Direction
• How STRONG the force is.
• Which way the force is
acting
Important:
• Objects do not change their motion
unless acted upon by an outside
force!
GRAVITY
• PREDICT IN YOUR NOTEBOOKS:
• When you drop them at the same time, what
will hit the ground first:
– A pencil , book, or ruler?
– Which will hit last??
GRAVITY
• You already know:
– Earth’s gravity pulls objects toward its center
without ever touching them.
– It’s why satellites stay in orbit, why pendulums
swing, and why apples fall off trees to the ground.
Gravity
• Maybe you know:
– Gravity is different on different planets.
– Your weight is a measure of the pull of gravity
between you and the Earth.
• It depends on your mass and the earth’s mass.
• It also depends on how far above the earth you are.
Gravity
• You probably DON’T know:
– Every object in the universe exerts a gravitational
pull on every other object, but only the gravity of
a large object (like earth) is strong enough to be
felt.
DO NOW
• Take this out from yesterday and continue
working if you need to.
• What are places you see
speed in everyday life?
• What do you know about
speed?
Point of Reference
• Motion is measured in relationship with some
location called a point of reference.
• Imagine riding on a train. You are not changing
position relative to the seat you are in, but you are
definitely changing position in relation to the
ground or the towns you are passing.
• What if you were walking toward the rear of
a train while the train is moving very slowly
forward?
• Imagine you are in Illinois. There is
a tornado moving at a speed of 50
mph in neighboring Indiana. Is this
enough information to know if you
need to evacuate your town?
NO!
• You would need to know not only the speed of
the tornado, but also the DIRECTION the
tornado is moving in!!!
Speed + Direction = velocity!
• When you know the speed AND the
direction of an object’s motion, then
you know the object’s VELOCITY!
Speed or Velocity?
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19.3 miles/hour
289 mph
3 meters/sec S
1.9 cm/s W
2,000 mi/h
Speed
Speed
velocity
velocity
speed
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1. Gravity
2. Magnetism
3. Electricity
4. Friction
• Magnetism is a force that pushes and pulls on
objects.
• Magnets have 2 poles – N and S
• Magnets exert magnetic force on objects
made of iron and certain other elements
• Opposites attracts
• Likes repel
• Objects get electrically charged when they
gain or lose ELECTRONS.
• Imagine an atom has 3 protons and 3
electrons. What is the OVERALL charge on the
atom?
• Then, it gains 2 electrons. Now, what is the
overall charge on the atom?
• Imagine you have an atom with 3 protons and
3 electrons and it loses 2 of its electrons. Now
what charge does it have???
• Force that results when 2 materials rub
against each other
• It goes AGAINST motion
– It slows it OR keeps it from starting
• Depends on 2 things:
– 1. texture of surface
– 2. how hard they press against each other
Types of Friction
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Air friction
Water / fluid friction
Sliding friction
Static friction (objects that are not moving)
What are sports where friction is
helpful?
• Soccer – cleats
What are sports where athletes
attempt to reduce friction?
• Swimming – suits, caps, body position to avoid
friction
• Ice skating
Gravity, Electricity, and Magnetism
• All can act at a distance and do not require
touching. BUT they are stronger when the
objects are closer.
• Magnetism and Electricity can be blocked, but
gravity cannot.
WORK – which of these scenarios is
the most work done and why?
• 1. holding a 20 pound backpack on your back
for ten minutes
• 2. lifting a 2 pound backpack from the floor to
the kitchen counter
• 3. running on a treadmill for 2 miles
WORK AND POWER
• Work, work, work. You might head off to your
job one day, sit at a computer, and type away
at the keys. That's all we do here. Is that
work?
• To a physicist, only parts of it are.
• Work is done when a force that is applied to
an object moves that object in the same
direction of the force.
• The work is calculated by multiplying the force
by the amount of movement of an object
• (W = F x d).
• Kayla exerts a force of 10 Newtons on her
science book. She moves her book 3 meters.
How much work did she do?
W=FxD
W = 10 Newtons x 3 meters
W = 10 N x 3 m
W = 30 N*m
Kayla does 30 Nm of work.
• A “Newton-meter” is quite a mouthful!
• Thus, scientists came up with a much easier
term to describe Newton-meters.
• 1 Newton-meter = 1 JOULE!
SOOO….
• Work is measured in
newton-meters, also
known as JOULES!
WORK , or NO WORK?
• Sitting and looking at a computer screen
* No work!
• Tapping on the keyboard and making the keys
move
•Yes work!
•You are applying a force and
making the keys move!
• Driving to your job
is not work because you just sit,
but the energy your car engine
uses to move the car does work.
• Imagine that you are holding a brick above the ground. Your
arm is straight out in front of you and it's pretty tough to hold.
Slowly, your arm gets tired, the brick feels heavier and
heavier, and you finally have to stop to let your arm rest. Even
though you put forth a lot of effort to hold the brick up, did
you do any work on the brick? Nope. The brick didn't move.
No work was done if no movement happened. If you lifted the
brick again after your arm had rested, that would be work.
DO NOW
• Morgan walks up the stairs, and it takes him
20 seconds. Her sister Lexi runs up the stairs
and arrives at the top in only 4 seconds.
• Who does more work? Why?
• Morgan and Lexi do the SAME
amount of work. Lexi, however, has
more POWER!
• She does the SAME amount of work in less time.
• Power = Work
Time
How can a person have more
power?
• 1. Do MORE work in the SAME time!
(ex: Lexi runs up 2 flights of stairs in the time it takes
Morgan to run up 1 flight of stairs)
Or
• 2. Do the SAME work in LESS time!
(Ex: Lexi runs up a flight of stairs in 4 seconds, but it takes
Morgan 20 seconds)
• Examples of Force
• If you were a ball sitting on a field and someone kicked
you, a force would have acted on you. As a result, you
would go bouncing down the field. There are often many
forces at work. Physicists might not study them all at the
same time, but even if you were standing in one place,
you would have many forces acting on you. Those forces
would include gravity, the force of air particles hitting
your body from all directions (as well as from wind), and
the force being exerted by the ground (called the normal
force).
• Let's look at the forces acting on that soccer ball before
you kicked it. As it sat there, the force of gravity was
keeping it on the ground, while the ground pushed
upward, supporting the ball. On a molecular level, the
surface of the ball was holding itself together as the gas
inside of the ball tried to escape. There may have also
been small forces trying to push it as the wind blew.
Those forces were too small to get it rolling, but they
were there. And you never know what was under the
ball. Maybe an insect was stuck under the ball trying to
push it up. That's another force to consider.
• If there is more than one force acting on an
object, the forces can be added up if they act
in the same direction, or subtracted if they act
in opposition. Scientists measure forces in
units called Newtons.
Write this down:
• All forces acting on an object make up the net
force.
• When all the forces acting on an object
balance, the object is in a state of equilibrium
SIR ISAAC NEWTON!
• There was this fellow in England named Sir Isaac Newton. A little
bit stuffy, bad hair, but quite an intelligent guy. He worked on
developing calculus and physics at the same time. During his
work, he came up with the three basic ideas that are applied to
the physics of most motion. The ideas have been tested and
verified so many times over the years, that scientists now call
them Newton's Three Laws of Motion.
Newton’s First Law of Motion
• An object at rest stays at rest, and an object
moving in a straight line at a constant speed
will continue to move in a straight line and at
a constant speed until a net force acts on the
object.
Inertia
• It is the tendency of an object to resist any change in motion.
• Ex: a pen sitting on your desk will continue to stay sitting on
your desk.
• Ex: a ball rolling across the ground will continue until it bumps
into something or until the force of friction makes it stop.
Inertia is influenced by:
• MASS! Objects with greater
mass have greater inertia than
objects with lesser mass.
Newton’s Second Law
• http://www.teachertube.com/viewVideo.php
?video_id=21643
Newton’s SECOND LAW
• Force = Mass x acceleration
• F=mxa
• What is acceleration
ACCELERATION
• It is a change of velocity in a unit of time.
• Speeding UP!, or
• Slowing down!, or
• Changing direction!
• Net forces acting on an object change its
velocity by causing it to speed up, slow down,
or change direction… all of which are referred
to as ACELERATION!!!
IMAGINE:
• You are pulling a 2 year old in a wagon. You
are going to start from stop….. And you are
going to ACCELERATE and speed up as fast as
you can.
• What force will produce the most
acceleration?
• What force will produce the least
acceleration?
F=mxa
• The larger the force….
• The smaller the force…
The greater the acceleration
The less the acceleration
Now… imagine!
• You are taking turns pulling a 2 year old in a
wagon, and then pulling Mr. Sammon in the
wagon.
• How does mass affect the force and the
acceleration???
• The larger the mass,
– The larger the force needed to have a certain
acceleration
– The smaller the acceleration will be if you use the
same force.
• The smaller the mass,
– The smaller the force needed to have a certain
acceleration
– The larger the acceleration will be for a given force
F = ma
• http://www.teachertube.com/viewVideo.php
?video_id=21643
DO NOW
• Give 2 examples of a time you see
Newton’s 1st law in action and 2
examples of times you see Newton’s
2nd law in action.
Newton’s third Law
• When one object exerts a force on another
object, the second object exerts a force on the
first object
• For every action (force) there is an equal and
opposite reaction (force).
• Action-reaction forces are always equal and
opposite, and they always occur in pairs
– See examples book page 424-425
– Remember from class: balloon, pushups, making a
bridge, holding a stack of books in your hands and
then someone lifts them
Newton’s 3 laws
• http://videos.howstuffworks.com/discovery/2
9382-assignment-discovery-newtons-first-lawvideo.htm