PHYSICS UNIT 2: DYNAMICS (Explaining Motion)

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PHYSICS UNIT 2 DYNAMICS(Explaining Motion)
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FORCES

• Force a "push" or a "pull
• unit Newtons, N (1 N is about ¼ lb)
• vector - includes direction
• contact forces and field forces (act over a
  distance)
• net force total effect of all forces acting on
  an object

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FORCES

• Typical Forces
• gravity, FG objects weight, always directed
  toward center of earth (FGmg mass
  acceleration due to gravity)
• normal force, FN supporting force a surface
  exerts on an object, always directed upward
  perpendicular to the surface
• tension, FT force transmitted by a rope or
  chain, directed along the rope, constant
  throughout the rope

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FORCES

• Free body diagrams show just one object the
  forces acting on the object (NOT forces the
  object is exerting on other things) example
  car hitting a wall

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Examples

• Apple on a table
• Rock under water
• Block on a hill
• Water skier
• Child pulled forward at an angle on a sled

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NEWTONS LAWS OF MOTION

• The Law of Inertia (1st Law) an objects
  velocity stays constant unless acted upon by a
  net external force
• inertia resistance to change in motion (mass is
  a measure of inertia, more mass more inertia)

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Example of Newtons 1st Law
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NEWTONS 2nd LAW OF MOTION

• The Law of Acceleration (2nd Law) a net force
  causes an acceleration proportional to the force,
  in the same direction, and inversely proportional
  to mass.
• Fnet ma Fnet sum of all forces or net force
  (N),
• m mass (kg),
• a acceleration (m/s2)
• 1 N 1 kgm/s2

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NEWTONS 2nd LAW OF MOTION

• Second
• The greater the force, the greater the
  acceleration
• The greater the mass, the greater the force
  needed for the same acceleration
• Calculated by F ma
• (F force, m mass, a acceleration)

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NEWTONS 3rd LAW OF MOTION

• The Law of Interaction (3rd Law) for every
  action force from one object on another, there is
  an equal magnitude, opposite direction reaction
  force from the 2nd object back on the 1st

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NEWTONS 3rd LAW OF MOTION

• Law of Interaction (3rd Law)
• action reaction forces do not balance each
  other - they are on different bodies (ex car
  pulling a trailer)
• equal force does not mean equal acceleration -
  depends on mass (ex person jumping off the
  ground)

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Examples of Newtons 3rd law
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FORCES

• Finding the Net Force (total of all forces on an
  object)
• draw a free body diagram
• identify label x y axes
• separate forces into x and y parts FxFcosq
  FyFsinq
• add all x forces, add all y forces
• equilibrium no net force x forces add up to
  zero, y forces add up to zero

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Example
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LAB 2.3 Elevator Scene 1
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LAB 2.3 Elevator Scene 2
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LAB 2.3 Elevator Scene 3
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LAB 2.3 Elevator Frame 1
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LAB 2.3 Elevator Frame 2
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LAB 2.3 Elevator Frame 3
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LAB 2.3 Elevator Frame 4
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LAB 2.3 Elevator Frame 5
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LAB 2.3 Elevator Frame 6
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LAB 2.3 Elevator Frame 7
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QUIZ 2.1

• Joe rolls a ball down a hill. The ball has a
  mass of 0.500 kg. The force pulling the ball
  down the hill is 6.00 N. The hill is 100.0 m
  long. (a) What is the balls acceleration? (b)
  How fast is the ball going at the bottom of the
  hill, if it started at rest at the top? (c) If
  the force on the ball doubled, what would happen
  to the balls acceleration? (d) If instead the
  mass of the ball doubled, what would happen to
  its acceleration?

12.0 m/s2
49.0 m/s
doubles (24 m/s2)
halves (6 m/s2)
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PHYSICS

• UNIT 2 DYNAMICS
• (Explaining Motion)

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NEWTONS LAWS OF MOTION

• Law of Inertia (1st Law)
• objects slow stop, or require continued force
  to keep moving, due to friction

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FRICTION

• Friction Force, Ff resistance to motion between
  objects in contact with each other
• acts parallel to contact surface, opposite to
  motion
• caused by uneven surfaces, molecular attraction

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FRICTION
kinetic friction lt static friction

• static friction resistance to starting motion
  (at rest)
• beneficial (walking, building, eating, wheels
  rolling)
• kinetic friction resistance to continued motion
  (sliding)
• undesirable (machines, moving furniture, wheels
  skidding)

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FRICTION

• coefficient of friction, m constant that depends
  on type of surfaces in contact
• ms coefficient of static friction
• mk coefficient of kinetic friction
• Ff mFN (friction force m normal force)

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FRICTION
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FRICTION

• on horizontal surface

• FN mg
• (normal force body weight)
• so Ff mmg

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FRICTION

• on tilted surface

q

• FN mgcosq
• so f mmgcosq

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PHYSICS

• UNIT 2 DYNAMICS
• (Explaining Motion)

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QUIZ 2.2

• A 1200 kg car sits on a horizontal road. (a) How
  much force does Joe need to push the car at a
  constant speed if the coefficient of kinetic
  friction is 0.600? (b) How much will the car
  accelerate if Joe uses a force of 10,000 N?

a) 7060 N
b) 2.45 m/s2
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PHYSICS

• UNIT 2 DYNAMICS
• (Explaining Motion)

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PROJECTILE MOTION

• Projectile motion parabolic trajectory (path)
• Two dimensions of motion horizontal (x),
  vertical (y)

vy vsinq
vx vcosq
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PROJECTILE MOTION

• Vertical Motion

if a bullet was fired horizontally, andanother
bullet was dropped from thesame height at the
same time, whichwould hit the ground first?
constant vertical acceleration due to
gravity(2nd Law)
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PROJECTILE MOTION

• A monkey hangs from a tree branch. A hunter aims
  his tranquilizer gun barrel straight at the
  monkey. When the hunter fires his gun, should
  the monkey keep holding on to the branch, or let
  go?

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PROJECTILE MOTION

• Vertical Motion
• position y h visinqit ½gt2
• a. for ground launch, h0, y visinqit ½gt2
• b. for horizontal cliff launch, q00, y h
  ½gt2
• speed vy visinqi gt
• flight time, T t when y0
• ground cliff

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PROJECTILE MOTION
A tank moving at constant speed fires ashell
straight up into the air. Where willthe shell
come back down?

• Horizontal Motion

• constant horizontal speeddue to no horizontal
  force(1st Law)

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PROJECTILE MOTION

• A snowmobile fires a flare, then slows down.
  Where does the flare land? If the snowmobile
  speeds up instead, where does the flare land?

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PROJECTILE MOTION

• Horizontal Motion
• position x vicosqit
• for horizontal cliff launch, qi0, x vit
• speed vx vicosqi
• range, R x when t T
• ground cliff

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PROJECTILE MOTION

• Example A projectile is launched from ground
  level with a velocity of 50 m/s at an angle of 30
  degrees. What is its position and velocity 2
  seconds later? What is its flight time? What is
  its range?

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PHYSICS

• UNIT 2 DYNAMICS
• (Explaining Motion)

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RELATIVE MOTION
A plane moving at constant speed drops a flare.
Describe the path of the flare.

• Reference Frames

projectile motion in one reference frame can be
vertical free fall in another reference frame
(and vice versa)
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PHYSICS

• UNIT 2 DYNAMICS
• (Explaining Motion)

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QUIZ 2.3

• Circle your answers! Watch sig. fig's units!
• 1. Joe throws a ball from ground level at an
  angle of 41º and a speed of 19 m/s. (a) Find the
  ball's vertical position after 1.5 seconds. (b)
  Find the ball's horizontal speed after 1.5
  seconds.
• 2. Jane throws a ball off a 95-m tall building
  horizontally at 19 m/s. (a) Find the ball's
  flight time. (b) Find the ball's range.
• y h visinqit ½gt2 vy visinqi
  gt
• x vicosqit vx vicosqi

7.67 m
14.3 m/s
4.40 s
83.6 m
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PHYSICS

• UNIT 2 DYNAMICS
• (Explaining Motion)

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UNIT 2 REVIEW

• Newton's Laws (Memorize!)
• 1st Law velocity stays constant unless acted
  upon by a net force
• 2nd Law net force mass x acceleration
• 3rd Law for every action force, there is an
  equal and opposite reaction force

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UNIT 2 REVIEW

• SF ma
• FG mg
• Ff mFN
• vf vi at
• Dx vit ½at2
• vf2vi2 2aDx

• y h visinqit ½gt2
• x vicosqit
• vy visinqi gt
• vx vicosqi