Work and Energy

1
Chapter 9

• Work and Energy

2
9.1 - Work, Power, Machines
Discussion!

• What is work?
• Imagine for a moment that your car has a flat
  tire. How do you change the tire?
• In the world of science, we have a lot of tools
  that help us to do work, plus a specific
  definition (and fancy equation and units!) for
  work.
• What are some tools that help us do work?

3
9.1 - Work, Power, Machines
Notes!

• Work - when force causes a change in the motion
  of an object in the direction of the force
• Work Force x Distance (W Fd)
• Standard units - Joule
• 1N m 1J 1kg m2/s2
• Put another way, 1 joule is 1 newton of force
  over the distance of 1 meter in the direction of
  the force

4
9.1 - Work, Power, Machines
Examples!

• How much work is done if someone pushes against
  the back wall, between the windows?
• How much work does a wrestler do in lifting his
  opponent 1.2m in the air, with an average force
  of 500N?
• A 75kg ninth-grader does 3 pushups in 3s, with an
  acceleration of 4m/s2. How much work is done, if
  each pushup is 1.5m?
• How much do you like math word problems?
• I hated them, too!

5
9.1 - Work, Power, Machines
Table exercises!

• A crane uses an average force of 5200N to lift a
  girder 25m. How much work is done by the crane
  on the girder?
• A 1N apple falls 1m. How much work is done on
  the apple by the force of gravity?
• Your bicycles brakes apply 125N of frictional
  force to the wheels as the bike travels 14m. How
  much work have the brakes done on the bicycle?
• A ninth-grade rower exerts a 165N force per
  stroke, while pulling the oar .8m. How much work
  does the rower do in 30 strokes?

6
9.1 - Work, Power, Machines
Discussion!

• Okay, that was work. What is power?
• If we asked each of the gentlemen in the room to
  come to the front and flex his muscles for us,
  would that be a demonstration of power? ?
• Compare the amount of work done if you
• Walk up a flight of stairs
• Run up the flight of stairs

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9.1 - Work, Power, Machines
Back to notes!

• Power - how much work is done in a certain amount
  of time
• Power Work / time (P W/t)
• Standard units - Watt
• 1W 1J/1s
• Another unit of power is horsepower (hp), the
  amount of power output of one draft horse. One
  hp is equal to 746W!

8
9.1 - Work, Power, Machines
More examples!

• It takes 100kJ of work to lift an elevator 18m.
  If this happens in 20s, what is the average power
  of the elevator during this process?
• A ninth-grade rower does 3960J of work in one
  minute. What is the power output in watts?

9
The Power of Ninth-Graders

• For this exercise, we need groups of 3
• Take out a piece of paper (it will be collected)
• Copy the items to measure in this lab
• Each group take a meter stick and a stop watch
• Everyone will participate, taking turns as
  timers, walkers, and recorders for the group

10
review

• What is work?
• What is the formula for work?
• What is the unit for work?
• What is power?
• What is the formula for power?
• What is the unit for power?

11
9.1 - Work, Power, Machines
More table exercises!

• Every second, a coal-fired power plant produces
  enough electricity to do 9x108J of work. What is
  the plants output in watts?
• Using a jack, thieves do 5350J of work to lift a
  car .5m to put it on blocks in 50s. What is
  their power output?
• A ninth grader is moving a 300N box of books.
  Calculate the power if the 9th grader
• Pushes with 60N of force 12m in 20s
• Lifts the box 1m onto a truck in 3s

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9.1 - Work, Power, Machines
Discussion slide!

• Which is easier, using Jack to lift the back of
  the car or using a jack to do the job?
• You do the same amount of work to raise the jack!
• Why do you think that is?
• How do machines, like the jack, help us to do
  work?
• Machines help us to multiply the force, by
  changing the direction of the input force or
  increase an output force by changing distance
  over which force is applied

13
9.1 - Work, Power, Machines
Notes!

• Mechanical advantage - a number that describes
  how much the force or distance is multiplied by a
  machine
• Equals the ratio between output and input forces
• Equals the ratio between input and output
  distances
• Mechanical advantage
• Output force / input force
• Input distance / output distance

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9.1 - Work, Power, Machines
Notes!

• Mechanical advantage (continued)
• Mechanical advantage gt1 multiplies the input
  force this will help to move heavy objects
• For example, calculate the mechanical advantage
  of a ramp that is 5m long and 1.5m high
• Input distance 5m output distance 1.5m
• Mech. Adv. 5m / 1.5m 3.3
• Mechanical advantage lt1 does not multiply force
  but increases distance and speed
• For example, swinging a baseball or softball bat,
  your arms and bat form a machine that increases
  speed without multiplying force

15
9.1 - Work, Power, Machines
Examples!

• A ninth-grader pulls on the handle of a hammer
  with a force of 15N. If the hammer has a
  mechanical advantage of 5.2, how much force is
  exerted on a nail in the claw?
• The bus driver exerts 55N of force on the
  steering wheel, which in turn applies 132N of
  force on the steering column. What is the
  mechanical advantage of the steering wheel?

16
review

• Without looking at your notes
• Describe mechanical advantage
• What is the formula for calculating mechanical
  advantage?
• What are the standard units for mechanical
  advantage?

17
9.1 - Work, Power, Machines
Table exercises!

• Calculate the mechanical advantage of a ramp that
  is 6m long and 1.2m high
• A ninth-grader pulls an oar handle .8m on each
  stroke. If the oar has a mechanical advantage of
  1.5, how far does the oar blade move through the
  water on each stroke?
• A student (obviously not a ninth-grader) uses a
  rope and pulley to raise the stage curtain, which
  weighs 140N. If the student pulls with a force
  of 140N on the rope, what is the mechanical
  advantage of the pulley?

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9.2 - Simple Machines
Notes!

• The most basic machines are called simple
  machines
• Other machines are either modifications or
  combinations of simple machines
• Two families
• Levers simple lever, pulley, wheel axle
• Inclined planes simple inclined plane, wedge,
  screw

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9.2 - Simple Machines
Discussion!

• How do levers work?
• What is a fulcrum?
• Can you name some examples of lever machines and
  where they have their fulcrums? Fulcra?!?

20
9.2 - Simple Machines
Back to notes!

• All levers have a rigid arm that turns on a
  fulcrum
• First-class lever most common type fulcrum in
  middle, input force applied down, output force up
  on opposite end
• Examples hammer, see-saw
• Second-class lever fulcrum on one end, input
  force on other end and applied upward, output
  force in middle
• Examples wheelbarrow, car jack

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9.2 - Simple Machines
More notes!

• Third-class lever fulcrum on one end, input
  force in middle and applied upward, output force
  on far end
• Examples flexing forearm many other body
  joints
• Pulleys - modified levers middle of pulley is
  the fulcrum of a first-class lever, with the rest
  of the pulley behaving like the rigid lever arms

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9.2 - Simple Machines
Short discussion!

• Examples of ramps?
• Would you rather push someone in a wheelchair up
  the ramp or up the stairs to enter Central?
• How about getting to your seat at the Linc?
• Do you miss the math?

23
9.2 - Simple Machines
Wedgies!

• Wedge - a modified inclined plane, acting like
  two planes back to back. Wedges turn a single
  downward force into two forces directed toward
  the sides
• Examples include wood splitters, axes, and nails

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9.2 - Simple Machines
Screws!

• Screw - an inclined plane wrapped around a
  cylinder
• Examples include screws, jar lids, spiral
  staircases
• Tightening a screw with gently sloping threads
  requires a small force acting over a long
  distance
• Tightening a screw with steeper threads requires
  greater force

25
review

• Without looking at your notes.
• What are the two families of simple machines?
• How many classes of levers are there?
• Describe the input and output forces of each
  class of lever (I like this as a test question!)
• In what direction do the forces from wedges go?
• What is a screw?

26
Correction!

• A ninth-grader pulls an oar handle .8m on each
  stroke. If the oar has a mechanical advantage of
  1.5, how far does the oar blade move through the
  water on each stroke?
• We calculated this, incorrectly, as 1.2m
• Correct 1.5 .8m / x
• x .8m/1.5 .53m
• Why was this a bad question?

27
9.2 - Simple Machines
Notes!

• Compound machines - a machine made up of two or
  more simple machines
• For example, scissors use two first-class levers
  joined at a common fulcrum each lever arm has a
  wedge that cuts the paper
• What other examples of compound machines can you
  think of?

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9.2 - Simple Machines

• What examples of compound machines can you find
  in this picture?

Source http//img.alibaba.com/photo/11496573/High
_Modulus_Carbon_Fibre_Bicycle_Carbon_Triathlon_Bik
es.jpg
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9.3 - Energy
Discussion!

• What is energy?
• How do you think energy differs from work?
• Describe some forms and examples of energy.
• If someone says that ninth-graders have too much
  energy, what do they mean?

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9.3 - Energy
Demonstration!

• If you hold a ball at arms length over your
  table, does it have any energy?
• What happens if you drop the ball?
• How many times did it bounce before it came to
  rest or began rolling?
• What about a ball on the floor - what kind of
  energy does it have?
• What if I hit it with a golf club?

31
9.3 - Energy
Notes!

• Energy - a measure of the ability to do work
• Standard units - joules
• Whenever work is done, energy is transformed or
  transferred to another system
• So, if I use a golf club or a hockey stick to hit
  a ball, I transfer energy from the moving object
  to the still object

32
9.3 - Energy
Notes!

• The amount of energy can be measured by how much
  work is done on the object
• Two main types of energy
• Potential
• Kinetic
• What do you know about potential and kinetic
  energy?

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9.3 - Energy
Notes!

• Potential energy - stored energy because of the
  relative position of objects within a system
• Examples Newtons apple has gravitational
  potential energy a rubber band has elastic
  potential energy
• Gravitational potential energy depends on mass
  and height. Why do you think this is so?
• Two apples, same size, different heights - which
  will hurt more if it hits you?
• Two apples, different sizes, same height - which
  will hurt more? Why?

34
9.3 - Energy
Notes!

• Potential energy - continued
• Gravitational PE mass x free fall acceleration
  x height (PE mgh)
• Remember, g 9.8m/s2 down
• Most of the time, h is measured from the ground
• Examples
• A 65kg ninth-grade rock climber goes 35m up a
  cliff from the ground. What is the gravitational
  potential energy of the climber?
• A diver has 3400J of PE after stepping onto a
  platform 6m above the water. What is the divers
  mass?

35
review

• Without looking at your notes
• What is energy?
• What is potential energy?
• What is the standard unit for energy?
• What is the formula for gravitational potential
  energy?
• How many notes on kinetic energy should we have
  today?

36
9.3 - Energy
Table exercise!

• Calculate the gravitational potential energy for
• A 1200kg car at Broad Olney, 42m above Ogontz
  Olney
• A 65kg ninth-grader on top of 8800m Mt. Everest
• A ninth-grader holds a 55g egg out the window,
  with a PE of 8J. How far from the ground is the
  egg?
• The water behind the Schuylkill River dam at
  Boathouse Row has a weight of 6.3x1012N for its
  top meter. The dam holds the water 7m above the
  river below. What is the PE of the top meter of
  water behind the dam?

37
9.3 - Energy
Discussion!

• Yesterday, we talked about kinetic energy as the
  energy of motion
• What do you know about that kind of energy? Do
  you recall any formulas?
• What would go into a formula for motion?
• When would a falling apple do its work?

38
9.3 - Energy
Notes!

• Kinetic energy - energy of an object because of
  its motion
• Kinetic energy depends on the objects mass and
  speed
• Kinetic Energy 1/2 x mass x velocity squared
  (KE 1/2 mv2)
• Note KE depends on speed more than it does on
  mass - why?
• Example
• Graph a 1N apple falling as it accelerates from
  0, 2, to 8 m/s

39
9.3 - Energy
Table exercise (one of them is sneaky)!

• What is the kinetic energy of a 44kg cheetah
  running at 31m/s?
• Some ninth-graders go bowling. What is the mass
  of the bowling ball if it has 16J of KE and a
  velocity of 2m/s down the alley?
• Going home today, your 1500kg car moves at
  42km/h what is its kinetic energy?
• We have a BIG snowstorm. What is your speed in
  m/s if your mass is 55kg and you generate 300J of
  energy sledding down a hill?

40
review

• Without looking at your notes
• What is kinetic energy?
• What is the formula for kinetic energy?
• What is the standard unit for kinetic energy?
• What questions does anyone have on the table
  exercises we did last week?

41
9.3 - Energy
Discussion

• Weve now discussed potential and kinetic energy.
  What are some other forms of energy out there?
• Have you ever worked with chemicals, that either
  absorbed or produced heat?
• What about the transfer of energy - how does that
  happen?
• What about food webs - who can tell us about
  those?

42
9.3 - Energy
Notes!

• Mechanical energy potential kinetic
• Chemical energy - when chemical bonds are broken
  or formed, energy changes (this is a kind of
  potential energy)
• The sun - provides us with energy from the food
  we eat ultimately, the animals and plants we
  consume got their energy from the sun, as
  photosynthesis converts solar to chemical energy

43
9.3 - Energy
Notes!

• Solar energy - results from nuclear fusion
• Electricity - the flow of electrons (what are
  electrons?) through wires lightning is a form of
  electricity
• Light - energy that flows through space in
  electromagnetic waves
• Who can tell us the colors of the spectrum?
• Light toward the blue end has more energy than
  light toward the red end

44
9.3 - Energy
Group work

• As a table, design an experiment that we can do,
  at Central, to generate the highest possible
  amount of kinetic energy
• Guidelines
• Everyone in the group must participate
• Safety first!!!
• Materials may come from within the school or from
  home (no purchases or disturbing other classes)
• The experiment needs to be conducted on Centrals
  property
• Write up your experiment and turn it in I will
  review the designs and make suggestions before
  any of them goes forward (designs to be graded)

45
review

• Without looking at your notes
• Name some other forms of energy, besides
  potential and kinetic
• Which end of the light spectrum has more energy?
• Solar energy results from what process?
• What flows to form electricity?

46
9.4 - Conservation of Energy
Discussion!

• Who likes roller coasters?
• Who can explain how energy works within a roller
  coaster system?
• What happens to energy when a basketball player
  bounces the ball several times before taking a
  free throw?
• If we drop a rubber ball, will it bounce all the
  way to its original height? Why not?

47
9.4 - Conservation of Energy
Notes!

• Energy within a system does not disappear,
  although it may change form, from potential to
  kinetic and from kinetic to potential balls may
  exhibit elastic energy
• Some energy may be transformed into heat or noise
  from air resistance (non-mechanical)
• Lets take a quick look at a roller coaster graph
• From our basketball discussion, we know that the
  ball will not bounce back to you if it is merely
  dropped

48
9.4 - Conservation of Energy
Notes!

• Law of Conservation of Energy - energy cannot be
  created or destroyed at any given time, the
  energy within a system is the same
• Energy does not appear out of nowhere, but it can
  be added to a system
• Example someone bouncing a basketball
• Energy does not disappear, but it can change form
• Example subway producing heat on rails or
  encountering air resistance

49
9.4 - Conservation of Energy
Demonstration!

• Part 1
• How high will the pendulum swing on the other
  side from the X?
• How many swings did it take before the pendulum
  failed to reach its original height?
• Part 2
• Hold the pendulum at the X someone else hold a
  pencil at the intersection of the lines
• How high did the pendulum go? Why?
• Part 3
• Try putting the pencil at different points - how
  did that affect the pendulum?
• Use the law of conservation of energy to explain
  our observations in this demonstration

50
9.4 - Conservation of Energy
Discussion!

• How efficient are machines? For instance, when
  lifting a wheelbarrow, is all of the work done on
  the handles?
• What other reductions can you find in the amount
  of work machines help us to do?

51
9.4 - Conservation of Energy
Notes!

• Efficiency - how much useful work a machine can
  do, the ratio of useful work output to total work
  input
• Efficiency useful work output
• work input
• Expressed as a percentage 100 efficiency would
  produce as much useful work as input work
• No machine has 100 efficiency (why not?)

52
9.4 - Conservation of Energy
Example!

• A sailor raises a 140N sail using an old, squeaky
  pulley. He needs 180J of work on the rope to
  raise the sail 1m, which does 140J of work on the
  sail. What is the pulleys efficiency?

53
9.4 - Conservation of Energy
Table exercises!

• It takes 1200J of work on a jack to lift a car to
  change a tire. How much work must Jack do if the
  jack is 25 efficient?
• Ninth-graders determine that they need 1800J of
  work for a piano to go up a ramp. To overcome
  friction, they actually use 2400J. What is the
  efficiency of the ramp?
• A wind turbine has an efficiency of 37.5. If a
  wind gust does 125J of work on the blades, how
  much output work can the windmill do as a result
  of the wind gust?

54
9.4 - Conservation of Energy
Final slide of Chapter 9!

• What is perpetual motion?
• Have you heard of a perpetual motion machine?
• Perpetual motion machines exist only in theory.
  Because of friction, no machine can achieve 100
  efficiency, so a perpetual motion machine is
  impossible
• New technologies are getting closer to perpetual
  motion, by reducing the amount of non-mechanical
  energy that leaks from systems