Physics 2211A Todays Agenda

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Physics 2211ATodays Agenda

• Newtons Laws of Motion (1st and 2nd)

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(No Transcript)
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What Do Forces Do?

• EXAMPLE Block on frictionless surface pulled
  with constant force
• A force is caused by an agent
• and acts on an object. More
• precisely, the
• object and agent INTERACT.
• A force is a vector.

OBSERVATION 1 Object moves with constant
acceleration when pulled with constant force.
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Effect of Increasing the Force

• ADD Rubber Bands

ADD RUBBER BANDS
OBSERVATION 2 Acceleration directly
proportional to force
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Effect of Adding Objects
ADD MORE BLOCKS
constant stretch
Three Rubber Bands
rubber bands
OBSERVATION 3 Slope is inversely proportional to
amount of matter being accelerated


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REVIEWNet Force

• When several forces act on an object, they add.
• Adding forces means adding vectors.

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Connect Force and MotionNewtons Second Law

• The acceleration of an object is
  proportional to the net force acting on
  it.
• The constant of proportionality is called
  mass (more precisely, inertial mass), denoted
  m (unitskilograms (kg))
• The mass of an object is a constant property
  of thatobject, and is independent of external
  influences.
• Force has units of MxL / T2 kg
  m/s2 N (Newton)
• 1 pound 1 lb 4.45 N

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Three forces act on an object as shown. In which
direction does the object accelerate?

1
2
3
4
5
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How does an object move when it is left alone?

• Aristotle Objects are naturally at rest

Galileo, Newton Objects naturally move
at constant velocity
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Newtons First Law(Law of Inertia)

• An object moves with a constant velocity if and
  only if the net force on the object is zero.

Constant velocity means moving object
continues with same direction and same speed.
Rest is special case (zero velocity)
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Static Equilibrium (object at rest)
Dynamic Equilibrium (object moving with constant
velocity)
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Inertial Reference Frames and Newtons Laws

• A Reference Frame is the place you measure from.
• Its where you nail down your (x,y,z) axes!
• An Inertial Reference Frame (IRF) is one where
  Newtons Laws
• are valid.
• USE NEWTONS LAWS TO TEST
• FOR INERTIAL REFERENCE FRAMES

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Test for Inertial Reference Frame

• EXAMPLE Airplane parked on runway
• Ball placed on floor of plane no net forces act
  on ball.
• (Note an airplane cruising at constant velocity
  yields the same result)
• .

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Test for Non-inertial Reference Frame

• EXAMPLE Airplane taking off.
• Ball placed on floor of plane no net forces act
  on ball.
• .

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The earth as an Inertial Reference Frame

• Strictly speaking, an Inertial Reference Frame is
  not accelerating with respect to distant stars.
• The earth accelerates only a small amount
  compared distant stars (rotation about its axis,
  revolving around the Sun), so the earth is, to a
  good approximation, an Inertial Reference Frame.

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Force Misconceptions

• Misconception There is a Force of Motion.

Misconception Objects that are thrown, hit,
projected have a Force of (Throw, Hit, etc.)
Misconception Air pressure pushes down on
everyday objects.
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The Free Body Diagram

• A representation of ALL forces acting on an
  object
• Key STEPS
• IDENTIFY ALL FORCES
• DRAW ARROWS REPRESENTING ALL FORCES
• DRAW COORDINATE SYSTEM
• Additional STEPS (suggested by text)
• Represent object as a dot (particle model)
• Draw and label net force (use with caution!)

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The Free Body Diagram...

• Consider a plank leaning against a wall.
• What are the forces acting on the plank ?

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The Free Body Diagram...

• Isolate the plank and identify agents interacting
  with plank

• P plank (object)
• F floor (agent)
• W wall (agent)
• E earth (agent)

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The Free Body Diagram...

• Identify forces on plank and draw arrows to
  represent forces

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The Free Body Diagram...

• Choose coordinate system, represent object with
  dot.

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Free Body Diagram

• In this example the plank is not moving (not
  accelerating)
• So Newtons 2nd Law for the plank is
• The plank is in static equilibrium.

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Full Physical Representation Example (Ice Block
on Frozen LakeExample 4.5)
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Full Physical Representation (Skier Pulled
UphillExample 4.6)
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An elevator, suspended by a cable, moves downward
at constant speed. Which of the following BEST
represents the corresponding free body diagram of
the elevator? (Notation---Tcable tension, w-
weight)
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Suppose you are jumping vertically upward. When
you are in mid-air, (feet off the floor), which
of the following represents the FBD of your body?
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Suppose you are jumping vertically upward. While
you are still pushing off the floor, which of
the following represents the FBD of your body?
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Suppose you are jumping vertically DOWNWARD.
While you are still landing on the floor, which
of the following represents the FBD of your body?
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Tools Ropes Strings

• Can be used to pull from a distance.
• Tension (T) at a certain position in a rope is
  the magnitude of the force acting across a
  cross-section of the rope at that position.
• The force you would feel if you cut the rope and
  grabbed the ends.

T
cut
T
T
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Tools Ropes Strings...

• An ideal (massless) rope has constant tension
  along the rope.
• If a rope has mass, the tension can vary along
  the rope
• For example, a heavy rope hanging from the
  ceiling...
• We will deal mostly with ideal massless ropes.

T Tg
T 0
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Tools Ropes Strings...

• The direction of the force provided by a rope is
  along the direction of the rope

T
Since ay 0 (box not moving),
m
T mg
w
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Tools Pegs Pulleys

• Used to change the direction of forces
• An ideal massless pulley or ideal smooth peg will
  change the direction of an applied force without
  altering the magnitude

F1
F1 F2
F2
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Tools Pegs Pulleys

• Used to change the direction of forces
• An ideal massless pulley or ideal smooth peg will
  change the direction of an applied force without
  altering the magnitude

FW on S mg
T mg
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• A block weighing 20 N is hung from a rope
  attached to a scale. The scale is then attached
  to a wall and reads 20 N. What will the scale
  read when it is instead attached to another block
  weighing 20 N?

m
m
m
(B)
(A)
(1) 0 N. (2) 20 N (3) 40 N
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• Draw a Free Body Diagram of one
  of the blocks!!

T

• Use Newtons 2nd Lawin the y direction

m
a 0 since the blocks are stationary
w
FTOT 0
T - w 0
T w 20 N.
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• The scale reads the tension in the rope, which is
  T 20 N in both cases!

T
T
T
T
T
T
T
m
m
m
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Suppose you sitting inside a car that is
accelerating forward (to the right). Which of
the following represents the FBD of your body?
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Suppose you sitting inside a car that is moving
to the right at constant speed. Which of the
following represents the FBD of your body?
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Suppose you sitting inside a car that is moving
to the right at constant speed. Which of the
following represents the FBD of your body?
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Suppose you sitting inside a car that is moving
to the right and slowing down by braking. Which
of the following represents the FBD of the CAR?
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Recap

• FreeBody Diagrams (Chapter 4.7)
• Pulleys and Ropes
• For next time Quantitative Newtons Laws of
  Motion (Chapter 5)

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