S-30

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S-30

• This cat is not happy.
• He is in need of a hair
• dryer.
• List five sources of energy that might
  be
• able to produce
• electricity for him.

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Work and Energy

• AP Physics
• Chapter 6

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Work and Energy

• 6.1 Work Done by a Constant Force

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6.1 Work Done by a Constant Force

• Work the product of the magnitude of the
  displacement times the component of the force
  parallel to the displacement
• Or (dot product)
• Work is measure in Joules (Energy)

6-1
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6.1 Work Done by a Constant Force

• If Force an displacement are in the same
  direction
• Work is done, velocity increases, energy of the
  object increases
• If Force is opposite to displacement, negative
  work is done, energy decreases

6-1
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6.1 Work Done by a Constant Force

• If there is no motion, or the force is
  perpendicular to motion, no work is done, there
  is no change in velocity, there is no change in
  energy

6-1
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6.1 Work Done by a Constant Force

• As long as Force and displacement are parallel,
  work is done

6-1
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6.1 Work Done by a Constant Force

• Example A person pulls a 50 kg crate 40m along
  a horizontal floor by a constant force of 100N _at_
  37o. The coefficient of friction is 0.20. What
  is the work done by each force acting on the
  crate?
• Free body diagram

6-1
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6.1 Work Done by a Constant Force

• Example A person pulls a 50 kg crate 40m along
  a horizontal floor by a constant force of 100N _at_
  37o. The coefficient of friction is 0.20. What
  is the work done by each force acting on the
  crate?
• Work done by N?

By W?
6-1
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6.1 Work Done by a Constant Force

• Example A person pulls a 50 kg crate 40m along
  a horizontal floor by a constant force of 100N _at_
  37o. The coefficient of friction is 0.20. What
  is the work done by each force acting on the
  crate?
• Work done by F?

6-1
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6.1 Work Done by a Constant Force

• Example A person pulls a 50 kg crate 40m along
  a horizontal floor by a constant force of 100N _at_
  37o. The coefficient of friction is 0.20. What
  is the work done by each force acting on the
  crate?
• Work done by f?

6-1
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6.1 Work Done by a Constant Force

• Example A person pulls a 50 kg crate 40m along
  a horizontal floor by a constant force of 100N _at_
  37o. The coefficient of friction is 0.20.
• What is the net work done on the object?

6-1
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Work and Energy

• 6.2 Work Done by a Varying Force

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6.2 Work Done by a Varying Force

• Work is the area under a Force vs. displacement
  graph.
• If force changes at a constant
• rate,
• Otherwise we use calculus
• to calculate the area

6-2
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Work and Energy

• 6.3 Kinetic Energy, the Work-Energy Principle

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6.3 Kinetic Energy, the Work-Energy Principle

• Energy the ability to do work
• Sufficient for Mechanical Energy
• Kinetic Energy (translational) due to motion
• Equation
• Work-Kinetic Energy

6-3
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6.3 Kinetic Energy, the Work-Energy Principle

• Work-Kinetic Energy Theorem (Work-Energy
  Principle) the net work done on an object is
  equal to the change in the objects kinetic
  energy

Work-Kinetic Energy Physlet
6-3
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6.3 Kinetic Energy, the Work-Energy Principle

• A 1000 kg car traveling 26.7 m/s can brake to a
  stop in 20 m. What is the force applied by the
  breaks?
• Free Body Diagram?

6-3
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6.3 Kinetic Energy, the Work-Energy Principle

• A 1000 kg car traveling 26.7 m/s can brake to a
  stop in 20 m. What is the force applied by the
  breaks?
• Solve

6-3
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6.3 Kinetic Energy, the Work-Energy Principle

• A 1000 kg car traveling 26.7 m/s can brake to a
  stop in 20 m. If the car is traveling twice as
  fast, how long does it take to stop?

6-3
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S-31

• A rocket powered 2000kg truck can go from 0 to 27
  m/s in 3.5 s.
• A. What is the acceleration of the truck?
• B. What is the displacement of the truck?
• C. How much
• work was
• done on the
• truck?

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Work and Energy

• 6.4 Potential Energy

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6.4 Potential Energy

• Potential Energy due to position or
  configuration
• Gravitational Potential Energy (Ug) due to
  position above the earths surface

6-4
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6.4 Potential Energy

• Elastic Potential Energy due to the position of
  a spring
• Hookes Law
• Equation for Elastic Potential Energy
• kspring constant

Determining a Spring Constant
Elastic Potential Energy
6-4
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Work and Energy

• 6.5 Conservative and Nonconservative Forces

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6.5 Conservative and Nonconservative Forces

• Conservative Forces independent of pathway
  (gravity)
• Energy can be returned (conserved)
• Nonconservative Forces depends on pathways
  (friction)
• Energy can not be returned

Physlet
6-5
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Work and Energy

• 6.6 Mechanical Energy and its Conservation

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6.6 Mechanical Energy and its Conservation

• If no energy is lost to nonconservative forces
• We can expand that to include the types of energy
  we have
• Principle of Conservation of Mechanical
  Energy-energy just switches forms

6-6
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6.6 Mechanical Energy and its Conservation

• If energy is lost to nonconservative forces
• For example if the energy was lost to friction

6-6
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Work and Energy

• 6.7 Problem Solving Using Conservation of ME

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6.7 Problem Solving Using Conservation of ME

• List the types of energy before the reaction
• List the types of energy after the reaction
• Consider any non conservative forces

6-7
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S-32

• The 75 kg Henry (French)
• jumps off a cliff that is 102
• m high. Assuming that the
• bungee has a resting length
• of 40 m, follows Hookes
• Law, and stops the guy 3 m before he hits the
  surface, what is the elastic constant of the
  bungee cord?

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S-32

• An unfortunate 45 kg child never learned to slide
  on anything but his face. If his face and the
  dirt have a coefficient m0.4, and he is running
  at 11 m/s when he starts his slide how much work
  is done by friction by the time he comes to a
  stop?

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Work and Energy

• 6.8 Other Forms of Energy

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6.8 Other Forms of Energy

• Electric
• Nuclear
• Thermal
• Chemical

6-8
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6.8 Other Forms of Energy

• Law of Conservation of Energy The total energy
  is neither increased nor decreased in any
  process.
• Energy can be transformed from one form to
  another.

6-8
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Work and Energy

• 6.10 Power

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6.10 Power

• Power the rate at which work is done
• Measured in watts
• Often convenient to write in terms of force

6-9
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6.10 Power
6-9
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S-33

• The worlds strongest woman lifts 186 kg
• upward a distance of 0.75 m. Assuming that
• the mass accelerated upward from rest the
• whole distance in
• 0.44s,
• What is the work
• done by the dainty
• lady?
• How much power
• did she generate?

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S-34

• Mike is not a very impressive driver. He
• drives his 1500 kg minivan into the living room
• of his moms house. If the van was traveling
• at 20 m/s and came to a stop in 2 m, what is
• the average force on
• Mike and the van?

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S-35I can use the conservation of energy to
calculate changes in position or speed

• Tarzan (and his very 50s family) are out
• swinging on their vine. The vine is 45 m long
• and makes and angle of 10o
• to the vertical. If Tarzan
• (m105 kg) runs at 15 m/s
• and jumps on the vine, what
• will be the vertical angle at
• the highest point the vine
• reaches?

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S-36 I can use the conservation of energy to
calculate changes in position or speed

• Sven likes to ride his pogo stick
  really
• high. If he has a mass of 115
  kg,
• and manages to reach a maximum
• height of 13 m when the
  
• spring is compressed
  0.4 m,
• what is the constant of the spring?

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S-37 I can use the conservation of energy to
calculate changes in position or speed

• The worlds biggest
• swing drops 19
• stories (57 m). If our
• 150 kg chubby
• champion ran at 11 m/s to
• jump off the cliff, and the
• rope was 89 m long,
• what is his velocity at
• the bottom?

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S-38 I can use the conservation of energy to
calculate changes in position or speed

• Big Mouse Trap!

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S-39

• Dont mess with this
• dog. If he has a mass
• of 25 kg (all muscle)
• and hits the 5 kg
• pendulum going
• 7.2 m/s, what will be
• the maximum vertical
• angle the rope makes.
• The string is 8 m long.

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S-40I can relate transformations between kinetic
and potential energy

• Using your brilliant knowledge of energy,
• why has the style of the high jumping
• changed over the years from
• A. Scissor B. Straddle C. Flop

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S-41

• A 112 kg weasel running at 32 m/s trips and rolls
  into a ball. He rolls up a 45m long frictionless
  hill that makes an angle of 22o to the
  horizontal. At the top of the hill, falls off a
  cliff that is 120 below his starting point. He
  falls on a spring that compresses 1.5 m before
  shooting him back into the air. He passes his
  girlfriend who is sitting in a tree that is 81 m
  tall. What is his velocity as he passes his
  girlfriend?

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S-42

• This is a ridiculously
• huge rabbit
• May your brain be as
• large while you take
• your test!

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