Review

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Review
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http//astro.temple.edu/elenab/
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Describing MotionKinematics in one dimension
Kinematics is a part of mechanics, which is the
description of how objects move.
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Speed
Speed is a measure of how fast object moves,
measured by a unit of distance divided by a unit
of time.
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Instantaneous Speed
Average speed doesnt indicate the different
speeds that may have taken place during shorter
time intervals.
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Average Speed
average speed total distance traveled/ time
elapsed Total distance traveled average speed
X time
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• What is the average speed of a cheetah that
  sprints 100 m in 4 s? How about if it sprints 50
  m in 2 s?
• If a car moves with an average speed of 60 km/h
  for an hour, it will travel a distance of 60 km.
• (a) how far would it travel if it moved at
  this rate for 4 h?
• (b) for 10 h?
• 3. In addition to speedometer on the dashboard of
  every car is an odometer, which records the
  distance traveled. If the initial reading is set
  at zero at the beginning of a trip and the
  reading is 40 km one half hour later, what has
  been your average speed?
• 4. Would it be possible to attain the average
  speed in previous example and never go faster
  than 80 km/h?

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Velocity
When we describe speed and the direction of
motion, we are specifying velocity
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• With what two physical values is the race-car
  driver concerned? Speed or velocity?
• The speedometer of a car moving to the east reads
  100 km/h. It passes another car that moves to the
  west at 100 km/h. Do both cars have the same
  speed? Do they have the same velocity?
• During certain period of time, the speedometer of
  a car reads a constant 60 km/h. Does this
  indicate a constant speed or a constant velocity?

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• How might you estimate your speed if the
  speedometer in your car is broken?
• 2. Which of the following can be used to measure
  an average speed stopwatch, odometer, or
  speedometer? An instantaneous speed?

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Acceleration
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We can change the velocity of something by
changing its speed by changing its direction or
by changing its speed and its direction.
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Motion at Constant Acceleration
Acceleration is a rate of change , or change per
second of velocity. v at
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• Which has the greater acceleration, an airplane
  that goes from 1000 km/h to 1005 km/h in 10 s, or
  a skateboard that goes from 0 to 5 km/h in 1 sec?
• What is the acceleration of a race car that
  whizzes past you at a constant velocity 400 km/h?

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Acceleration on Galileo Inclined Planes
Galileo found greater accelerations for steeper
inclines. The ball attains max acceleration when
the incline is tipped vertically.
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• a ball rolling down an inclined plane is moving
  with constant acceleration
• greater accelerations for steeper planes, max
  acceleration when incline is tipped vertically
• regardless of weight and size, when air
  resistance is small enough to be neglected, all
  objects fall with the same unchanging
  acceleration.

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We call this acceleration the acceleration due to
gravity on the Earth, and we give it the symbol
g. g 9.80 m/ s2 10 m/ s2
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How Far?
With his inclined planes Galileo found that the
distance a uniformly accelerating object travels
is proportional to the square of the
time Distance acceleration x time x time /2.
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At a given location on the Earth and in the
absence of air resistance, all objects fall with
the same constant acceleration
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• Which of the following (if any) could not be
  considered an accelerator in an automobile gas
  pedal, brake pedal, steering wheel?
• A sports car accelerates from 65 mph to 75 mph in
  2 seconds while a minivan accelerates from 20 mph
  to 35 mph in 2 seconds. Which one has the larger
  acceleration?
• You are standing on a high cliff above the ocean.
  You drop a pebble, and it strikes the water 4
  seconds later. Ignoring the effects of air
  resistance, how fast was the pebble traveling
  just before striking the water? What is the
  height of the cliff?

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Explaining Motion
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Galileos inclined planes revisited
Slope upward speed decreases
Slope downward speed increases
No slope. Does speed change?
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Initial
Final position
Where is the final position?
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Newtons first law of motion
The first law incorporates Galileos idea of
inertia and introduces a new concept, force.
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Every object continues in its state of rest, or
of uniform motion in a straight line, unless it
is compelled to change the state by forces
impressed upon it.
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Examples

• Assume that you are pushing car across a level
  parking lot. When you are stop pushing, the car
  comes to stop. Does this violate Newtons first
  law? Why?
• Why does a tassel hanging from the rearview
  mirror appear to swing forward as you apply the
  brakes?

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If you were traveling toward a distant star and
you ran out of fuel, would your spaceship slow
down and stop? Explain.  
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Addition of Vectors Graphical Methods
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Examples

• In everyday use, inertia means that something is
  hard to get moving. Is this the only meaning it
  has in physics? If not, what other meaning does
  it have?
• How would you determine that two objects have the
  same inertia?
• When a number of different forces act on an
  object, is the net force necessarily in the same
  direction as one of the individual forces? Why?
• Modern cars are required to have headrests to
  protect your neck during collisions. For what
  type of collision are these headrests most
  effective?

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Newtons Second Law
The net force on an object is equal to its mass
times its acceleration and points in the
direction of acceleration Fnet ma
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Examples

• Forces of 4 N and 6 N act on the object. What is
  the minimum value for the sum of these two
  forces?
• Two ropes are being used to pull a car out of a
  ditch. Each rope exerts a force of 700 N on the
  car. Is it possible for the sum of these two
  forces to have a magnitude of 1000N? Explain your
  reasoning.
• If the net force on a boat is directed due east,
  what is the direction of the acceleration of the
  boat? Would your answer change if the boat had a
  velocity due north but the net force still acted
  to the east?

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Examples

• Describe the force(s) that allow you to walk
  across the room.
• You are analyzing a problem in which two forces
  act on an object. A 200 N force pulls to the
  right, and a 40 N force pulls to the left. Your
  classmate asserts that the net force is 200 N
  because that is the dominant force that is
  acting. What is wrong with that assertion?

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Net Force
The combination of forces that act on the object
is the net force
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Units of Force
1 newton (N) 1 Kg m/s2
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• You push on a crate that sits on a smooth floor
  and it accelerates. If you apply four times the
  pushing force, how much greater will be the
  acceleration?
• Same for a rough surface.

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Friction
If you apply a force to an object, a force of
friction usually reduces the net force and
resulting acceleration. The direction of
friction force is always in the direction
opposing motion. The force of dry friction
between solid surfaces doesnt depend on speed or
area of contact.
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Question

• A jumbo jet cruises at constant velocity of 1000
  km/h when the thrusting force of its engines is
  constant 100000 N.
• What is the acceleration of the jet?
• What is the force resistance on the jet?

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Mass and Force of Gravity
Mass The quantity of matter in an object. It is
also the measure of the inertia or sluggishness,
that an object exhibits in response to any effort
made to start it, stop it, or change its state
of motion in any way. Force of Gravity mg
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Newtons Second Law
The net force on an object is equal to its mass
times its acceleration and points in the
direction of acceleration Fnet ma
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Important acceleration of a body is always in
the direction of net force!!!!
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Free-body diagrams
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Free fall revisited
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When Acceleration is Less Than g Nonfree Fall
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Examples

• Forces of 4 N and 6 N act on the object. What is
  the minimum value for the sum of these two
  forces?
• Two ropes are being used to pull a car out of a
  ditch. Each rope exerts a force of 700 N on the
  car. Is it possible for the sum of these two
  forces to have a magnitude of 1000N? Explain your
  reasoning.
• If the net force on a boat is directed due east,
  what is the direction of the acceleration of the
  boat? Would your answer change if the boat had a
  velocity due north but the net force still acted
  to the east?

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Newtons Third Law of MotionForces and
Interactions
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In a broader sense, a force is not a thing in
itself but makes up an interaction between one
thing and another.
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Newtons Third Law of Motion
Whenever one object exerts a force on a second
object, the second exerts equal and opposite
force on the first. On every action there is
equal and opposite reaction
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Which force we call action and which we call
reaction doesnt matter. The point is that
neither exists without the other.
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Action and Reaction on Different Masses
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• A car accelerates along a road. Identify the
  force that moves the car.
• A high speed bus and an innocent bug have a
  head-on collision. The force of impact splatters
  the poor bug over the windshield. Is the
  corresponding force that bug exerts against the
  windshield greater, less or the same? Is the
  resulting acceleration of the bus greater than,
  less than or the same as that of the bug?

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• A skier is skiing down a steep slope, traveling
  at constant speed (that is, the skier has reached
  terminal velocity). What are the size and
  direction of the net force on the skier?
• A car can accelerate at 2 meters per second per
  second when towing an identical car. What will
  its acceleration be if the towrope breaks?
• If the number of different forces act on the
  object, is the net force necessarily in the same
  direction as one of the individual forces?

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• You apply a 75 N force to pull a childs wagon
  across the floor at constant speed. If you
  increase your pull to 80 N, will the wagon speed
  up to some new constant speed, or will it
  continue to speed up indefinitely? Explain your
  reasoning.
• You are riding an elevator from your tenth-floor
  apartment to the parking garage in the basement.
  As you approach the garage, the elevator begins
  to slow. What is the direction of the net force
  on you?

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• If the force exerted by a horse on a cart is
  equal and opposite to the force exerted by a cart
  on the horse, as required by Newtons third law,
  how does the horse manage to move a cart?
• A soft-drink sits at rest on a table. Which of
  the Newtons laws explains why the upward force
  of the table acting on the can is equal and
  opposite to Earths gravitational force pulling
  down on the can?
• A book sits at rest on a table. Which force does
  Newtons third law tell us is equal and opposite
  to the gravitational force acting on the book?

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Circular Motion
An object moving along a circular path at a
constant speed must have a net force acting on
it.
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It is important to distinguish between adjectives
centripetal (center-seeking) and centrifugal
(center-fleeing). The force, we are discussing ,
the centripetal force, is directed toward the
center of the circle.
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A Car, Rounding a Curve
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Velocity and acceleration are perpendicular each
other at each point of this circle.
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Dynamics of Uniform Circular Motion
S F ma m V2/R
Centripetal force is just a term, which means
that there is the acceleration pointed to the
center.
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Projectile Motion
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When something is thrown or launched near the
Earth surface, it experiences a constant
vertical gravitational force. Motion under these
conditions is called projectile motion.
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Important The study of projectile motion is
simplified because the motion can be treated as
two mutually independent, perpendicular motions,
one horizontal and the other vertical.
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Question
Suppose a bullet is fired horizontally from a
pistol and simultaneously another bullet is
dropped from the same height. Which bullet hits
the ground first?
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ImportantThe object projected horizontally will
reach the ground in the same time as the object
released vertically from the rest.
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Fast Moving Projectiles Satellites
The Earth satellite is simply a projectile that
falls around the Earth rather than into it.
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Question
A newspaper report reads in part, The space
shuttle orbits Earth at an altitude of nearly 200
miles and is traveling at a speed of 18,000 mph.
The shuttle remains in orbit because the
gravitational force pulling it toward Earth is
balanced by the centrifugal force (the force of
inertia) that is pulling it away from Earth.
Explain why this newspaper should hire a new
reporter.
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Questions

• What is the force that allows a person on
  roller-blades to turn a corner? What happens if
  this force is not strong enough?
• A child rides on a carousel at constant speed. In
  which direction does each of the following
  vectors point?
• a. velocity
• b. change in velocity
• c. acceleration
• d. net force.

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Questions
A vine is just strong to support Tarzan when he
is hanging straight down. However, when he tries
to swing from tree to tree, the same vine breaks
at the bottom of the swing. How could it happen?