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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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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(No Transcript)
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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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Free Body Diagram

• FBD A vector Diagram that shows the forces that
  act on a system. It is used to determine Fnet .
• Once you determine Fnet use Newtons second Law
  to solve the Physics problem.

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Equilibrium

• The case where acceleration is zero. If a system
  is in equilibrium all of the forces are balanced.
  This allows you to determine forces that act on
  an object.

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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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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

• 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

• 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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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