Warmup

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Warmup
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Force

• Affects on Velocity and Acceleration

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Force Causes Change in Velocity
Oh! Skeeter!
Ha, ha, good one
This one time at band camp
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Force and Motion

• Newtons First Law
• At rest Stays at rest (until force is applied)
• In motion Stays in motion (until force is
  applied)
• Force causes change in velocity
• Force causes acceleration
• Force causes a change in direction

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Warm-up

• A blue one with a nubbin was moving at 5m/s
  straight down when the problem started. The
  difference between the bottom and the top is 3
  times the height of a trans-atlantic 10m
  building. This nubbin sporting thing is
  earthbound. (include units)

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Types of Fundamental Force

• Gravitational Force
• Force we use in this section
• Electromagnetic Force
• Includes the contact forces we work with in this
  section
• Nuclear Force
• Weak Force
• (summarize)

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Electromagnetic Force

• Contact Forces
• Normal
• Friction
• Static friction when the object is not in motion
• Sliding friction when the object is in motion
• Tension
• Spring
• (Use article to organize with topic oval)

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Force

• Newtons Third Law
• Every action has an opposite and equal reaction.

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Force

• Experiment with force sensors.
• equal and opposite

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Key Vocab

• Net force (Fnet)
• Vector addition/subtraction
• 5 N 7N
• The resultant is the net force
• 5N 7N
• 12N

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Newtons Second Law

• Two men pull a 50-kg box with forces 19.7 N and
  15.6 N in the directions shown below. Find the
  net force of the box.
• 19.7 N
  15.4 N
• or
• -19.7N

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Force

• The pound-force or simply pound (abbreviations
  lb, lbf, or lbf) is a unit of force
• 1N0.225lb 1lb4.45N
• Normal plus lift (Fnet)
• Weight from force gauge.
• Upward force must be greater than gravity to have
  upward acceleration.
• HW
• 16,17, (pg 97),
• Example problem 2 pg 99 with elevator 3 times
  with a 2.50, 3.00, 15.0m/s2 and t1.50, 3.25,
  3.00s
• 19, 20, 22, 25 (pg 100 101)

Demo
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Force

• Newtons Second Law...........
• Weight is a Force...........
• Defined with the universal constant G.

1 lbf 4.448222 N
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Force and Gravity
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Key Vocab

• Normal Force
• Force due to gravity and mass is referred to as a
  normal force or the normal vector.
• Normal vector also refers to a vector that
  intersects at 90 degrees.
• (also stated as A vector that is
• perpendicular to the tangent
• line at the interface)

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Newtons Second Law

• Two men pull a 50-kg box with forces 19.7 N and
  15.6 N in the directions shown below. Find the
  resultant acceleration of the box and the
  direction in which the box moves.
• 19.7 N
  15.4 N
• or
• -19.7N

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Practice -

• A large helicopter is used to lift a heat pump to
  the roof of a new building. The mass of the
  helicopter is 7.0x103 kg and the mass of the
  heat pump is 1700 kg.
• a. How much force must the air exert on the
  helicopter to lift the heat pump with an
  acceleration of 1.2 m/s2?
• b. Two chains connected to the load each can
  withstand 95,000 N. Can the load be safely lifted
  at 1.2 m/s2?

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Air

• Drag Force exerted by a fluid on an object
  moving through the fluid.
• Terminal Velocity drag force is equal to force
  of gravity

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Friction
Friction Coefficients Table 5.1 pg 129

• Static friction Ff,s
• Friction when there is no motion between the
  objects.
• Ff,s lt usFN
• Sliding friction (or kinetic friction) Ff,k
• Friction when surfaces are rubbing against each
  other (in motion).
• Ff,k ukFN

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Friction
Normal

• A wood block on a wood plank.
• m kg
• us 0.5
• uk 0.2
• FN N

Friction
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Review Elevator Example

• Fnet FE (-Fg)
• Fnet ma (represents the force on the
  system as a whole)
• FE Fnet Fg

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Tension

• Force exerted by pulling (usually a string or
  rope).
• For now, consider strings, ropes and pulleys
  (also called sheaves or blocks) to be massless
  and frictionless.
• Provide a change in direction of force.

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Tension Problem

• The blocks shown are placed on a smooth
  horizontal surface and connected by a piece of
  string. If a 8.8-N force is applied to the 8.8-kg
  block, what is the tension in the string?

6.1N
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Tension

• Three blocks A, B, and C are connected by two
  massless strings passing over smooth pulleys as
  shown below, with the 3.4-kg block on a smooth
  horizontal surface. Calculate the tension in the
  strings connecting A and B, and B and C.

54N 48N
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Tension
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Tension Practice
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Sketch Problem
68N
40N
A
C
B
68N
40N
ABC
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Sketch Problem
68N
40N
A
C
B
40N
68N
C
AB
68N
40N
A
BC
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Fnet

• Fnet sum of all forces acting on a system ma
• Fnet Ff,k FT Fpush Fgravity ma

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Force

• Newtons Third Law
• Every action has an opposite and equal reaction.

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Force

• Newtons Second Law...........

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Force

• Newtons first law
• When the net forces are zero, an object at rest
  remains at rest and an object in motion remains
  in motion in the same direction at the same
  speed.
• For Fnet 0
• velocity is constant v1 v2
• acceleration is zero

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Friction
Friction Coefficients Table 5.1 pg 129

• Static friction Ff,s
• Friction when there is no motion between the
  objects.
• Ff,s lt usFN
• Sliding friction (or kinetic friction) Ff,k
• Friction when surfaces are rubbing against each
  other (in motion).
• Ff,k ukFN

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Friction

• Static friction Ff,s
• Friction when there is no motion between the
  objects.
• Ff,s lt usFN
• Ff,s lt usmg

10N
5kg
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Review Elevator Example

• Fnet FE (-Fg)
• Fnet ma (represents the force on the
  system as a whole)
• FE Fnet Fg

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Setting up the Problem

• Connected by a massless string, pulled along a
  surface with a coefficient of friction

100N
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Steps

• What forces are present?

• What is the value of each force?

• Draw the free body diagram

100N
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Steps

• List known variables, solve for unknown

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Break apart

• Find the tension in each string
• First, identify the forces of each part

100N
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Consider each section as a system

• Draw a free body diagram for this system

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Consider each section as a system

• Draw a free body diagram for this system

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Break apart

• Draw a sketch (free body diagram) for each string

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Trig Identities

• SOHCAHTOA
• Adjacent Leg Hypotenuse cos?
• Opposite Leg Hypotenuse sin?
• Opposite Leg Adjacent Leg tan?

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Force components

• The magnitude of the force 1 is 87 N, of force 2
  is 87 N, and of 3 is 87 N. The angles ? 1 and ?
  2 are 60 each. Use the Pythagorean theorem and
  trig identities to find the resultant of the
  forces 1, 2 , and 3 .

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Drag Force

• Terminal velocity is when an object in free fall
  has reached equilibrium. That is, the drag force
  is equal to the force of gravity.
• The Fnet for a system in equilibrium is always
  zero.

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Friction with Force components

• The system shown below is in equilibrium.
  Calculate the force of friction acting on the
  block A. The mass of block A is 7.10 kg and that
  of block B is 7.30 kg. The angle ? is 48.0.

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Break Apart

• The system shown below is in equilibrium This
  means Fnet0, since Fnetma, then ma0 and a0
• In order for the net force to be zero on block A
  (no acceleration) the tension in the rope pulling
  block A has to be matched by the friction between
  block A and the surface.

The tension here must be equal to the friction
resisting motion
The friction here must be equal to the tension
pulling A
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Break Apart

• There is a force from gravity acting on block B.
  
• Convert this force to its x and y components.
• Using the concept of equilibrium, realize that
  the sum of x components is 0

? is 48.0.
7.30kg
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Two wire tension
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Familiar Problem

• Three blocks A, B, and C are connected by two
  massless strings passing over smooth pulleys as
  shown below, with the B block on a smooth
  horizontal surface.
• A2kg, B3kg, C5kg

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Redraw the problem
Force of gravity on A
Force of gravity on C
A
C
B

• The problem may be redrawn to help visualize
  which direction the forces are acting and which
  direction is positive.

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Breaking the problem down

• Find the total mass of the system ABC.
• Find the net force of the system ABC.
• Find the acceleration of the system ABC.
• Find the acceleration of the system C.
• Find the total mass of the system C.
• Calculate the force due to the acceleration of
  system C.
• Calculate the force due to gravity on system C.
• Calculate the tension in the strings connecting B
  and C.

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Spring (restoring force)

• F-kx
• kconstant
• xdisplacement

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Force

• Experiment with force sensors.
• equal and opposite
• normal plus lift (Fnet)
• friction (static and sliding)

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