Force and Motion

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Force and Motion

• Chapter 12

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I. Forces

• A. A force is a push or a pull that acts on an
  object
• B. Can make resting objects move
• C. Can accelerate moving objects
• (change speed or direction or both)

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• D. SI units is the Newton
• 1. A Newton is the force needed to accelerate a 1
  kg mass 1 m/s2

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II. Combining forces

• A. Balanced forces
• 1. equal and opposite in direction
• ex. Tug of war
• ?----- -----?
• 2. net force is zero

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• B. Unbalanced forces
• 1. one force is greater than another
• 2. net force is NOT zero
• 3. when unbalanced forces act on an object,
  the object accelerates
• 4. ? ? --?
• ? ?---- ?--

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III. Friction

• A. Friction is a force that opposes the motion of
  objects that touch as they move past each other.
• B. Depends on two things
• 1. Type of surface rough vs. smooth
• 2. amount of force pressing things together

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• C. Four kinds of friction
• 1. Static friction
• a. force that acts on objects that are not
  moving
• b. always in opposite direction to applied
  force
• ex. taking a step

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• 2. Sliding friction
• a. force that opposes direction of motion of
  an object as it slides over a surface
• b. less than static -- need less force to keep
  object moving than to get it started

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• 3. Rolling friction
• a. force acting on rolling objects
• 100-1000 times less than the previous 2
• b. ex. using dollies to move big objects,
  ball bearings

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• 4. Fluid friction
• a. opposes motion in fluids
• (air and liquids)
• b. ex. stirring cake batter, air resistance

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IV. Gravity

• A. Gravity is an attractive force that acts
  between two masses
• B. Exerted by every object on every other object
• C. Depends on two things
• 1. mass of objects
• more mass more gravitational pull
• 2. distance between objects
• greater distance less force

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V. Falling objects

• A. Gravity causes objects to accelerate downward
• B. Air resistance acts in the opposite direction
  and reduces acceleration

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• C. Acceleration due to gravity is 9.8 m/s/s on
  Earth
• 1. drop a bowling ball and golf ball from a
  bridge they hit the water at the same time
• 2. Not true for all objects because of air
  resistance
• ex. ball hits before sheet of paper

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• D. Can calculate velocity of falling objects
• 1. acceleration due to gravity x amount of
  time it falls
• 2. ignores air resistance

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• E. Terminal velocity
• 1. highest velocity reached by a falling
  object
• 2. air resistance increases as speed
  increases
• 3. terminal velocity is reached when force of
  air resistance force of gravity

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VI. Projectile Motion

• A. Projectile an object thrown or shot through
  the air
• (not under its own power)
• ex. bullet, baseball

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• B. Motion of projectile
• 1. Makes a curved path
• a. Horizontal motion
• 1. produced by bow, arm, bat etc
• 2. parallel to Earth
• 3. velocity is constant

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• b. Vertical motion
• 1. produced by gravity
• 2. vertical velocity increases (9.8 m/s/s)

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• C. Vertical and horizontal motion are independent
  of each other
• a. ball thrown horizontally
• b. ball dropped from same height
• c. both hit ground at the same time

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VII. Newtons First Law

• A. An object in motion stays in motion and an
  object at rest stays at rest unless a net force
  acts on it
• B. The state of motion of an object does not
  change as long as the net force acting on the
  object is zero

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• C. Also called The Law of Inertia
• 1. inertia tendency of an object to resist
  a change in its motion
• D. Examples
• 1. hockey puck on ice
• 2. car crash

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VIII. Newtons Second Law

• A. Acceleration of an object is equal to the net
  force acting on it divided by the objects mass
• 1. Fma
• larger the force, greater the acceleration
• larger the mass, greater force needed to
  accelerate
• easier to push a small car than a big car?
• car goes faster when 2 people pushing car or 10
  people?

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• B. Acceleration is always in the same direction
  as net force.
• C. Example problem
• 1. A box has a mass of 150 kg. If a net force of
  3000 N acts on the box, what is the boxs
  acceleration?

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IX. Weight and Mass

• A. Mass is a measure of the amount of matter in
  an object.
• 1. It measures the inertia of an object

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• B. Weight is a measure of the force of gravity on
  an object
• 1. depends on mass and gravity
• 2. greater the mass, the greater that gravity
  acts on it, so it weighs more
• 3. Earth has more mass than the moon
• so more gravity here -- so weigh more on Earth
• 4. less gravity, less weight

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• C. Weight changes with gravity BUT MASS DOES NOT
  CHANGE
• D. Weight mass x gravity

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X. Newtons Third Law

• A. To every action there is an equal and opposite
  reaction
• B. Action and Reaction forces
• C. Examples
• 1. pushing off wall on a rolling chair
• 2. swimming

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XI. Momentum

• A. momentum mass times velocity
• pmv
• m is mass in kg
• v is velocity in m/s
• p is momentum in kg m/s
• triangle

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• B. Greater the mass of the moving object the
  greater the momentum
• C. Greater the velocity of the moving object the
  greater the momentum

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• C. Example problem
• 1. Compare the momentum of a 7.3 kg bowling ball
  traveling at a velocity of 1.1 m/s to a .014 kg
  bullet at 850 m/s

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XI. Conservation of momentum

• A. If no net force acts on a system (group of
  objects), the total momentum of the system does
  not change.
• B. Examples
• 1. Look on p. 376
• 2. bowling ball and pins

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XII. Universal Forces

• A. Electromagnetic forces
• 1. Can both attract and repel
• 2. two types
• a. electric forces
• between protons and electrons
• ex. Static

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• b. magnetic forces
• two poles
• attract or repel

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• B. Nuclear forces
• 1. Two types
• a. strong nuclear forces
• hold protons and neutrons together
• b. weak nuclear forces
• affect the whole atom

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• C. Gravitational forces
• 1. every object is attracted to every other
  object
• 2. weakest universal force but most effective
  over long distances