Work and Energy

1
Chapter 6

• Work and Energy

2
Work

• Work is the product of a force acting over a
  distance
• More simply stated W f x d
• F is measured in Newtons distance is meters
• N x m is called a Joule or J
• We also use kJ or MJ for objects that produce
  large amounts of work

3
Concept Check

• How much work is needed to lift an object that
  weighs 500 N to a height of 4 meters?
• 500 N x 4 m 2000 N m or 2000 J
• How much work is needed to lift it twice as high?
• 500 N x 8 m 4000 J
• How much work is needed to lift 1000 N to a
  height of 8 m?
• 1000 N x 8 m 8000 J

4
Power

• Power is the rate at which energy is changed from
  one form to another
• It is the rate at which work is done
• P W / t
• The unit is J / s or Watt or W
• Note that I will cross the tops of my Watts in
  problems to make them look different than the W
  for work
• Again, we use kW or MW for large amounts of Power

5
Concept Check

• You do work when you do push ups. If you do the
  same number of push ups in half the time, how
  does your power output compare?
• You should have 2 x the power
• How many watts of power are needed when a force
  of 1 N moves a book 2 m in a times of 1 second?
• W F x d 1 N x 2 m 2 J and P W / t
• P 2 J / 1 s 2 W

6
Energy

• Energy is the ability to do work
• Mechanical energy usually has motion like a
  bouncing ball
• A ball stops moving because its energy is
  converted from one form to another
• In this case, the mechanical energy is turned
  into a different form of energy such as heat and
  the compression of the balls air
• Energy is measured in J because it is associated
  with work

7
Concept Check

• Person A pushes a block of ice up a ramp five
  times longer than Person B lifts its straight up
  to the same height. How much more force does
  person B exert when he lifts the ice?
• 5 x the force
• Who does more work on the ice?
• They both do the same because the distance up was
  the same in the end

• If both jobs are done in the same time, who
  expends more power?
• They both do the same. Same work / Same time is
  Same Power

8
Potential Energy

• It is the energy that arises because of an
  objects position stored energy
• PE is a form of mechanical energy
• PE can get complicated such as calculating how
  much energy is stored in a bungee cord
• In this class, we will only do calculations for
  Gravitational Potential Energy

• GPE weight x height
• GPE m x g x h mgh
• We only use the height straight up in meters and
  not the path it takes to get there
• On the last slide, both person A B resulted in
  the same vertical change of height but their
  paths were different
• The unit for PE is Joules

9
Concept Check

• How much work is done in lifting a 200 N block of
  ice a vertical distance of 2.5 m?
• W F x d 200 N x 2.5 m 500 J
• How much work is done in pushing the same block
  of ice up a 5 m long ramp? The force needed is
  only 100 N (advantage of inclined plane).
• W F x d 100 N x 5 m 500 J
• What is the increase in the blocks PE in each
  case?

• Remember N is m x g or what we call weight
• Weight is the force that gravity pulls on a
  quantity of matter
• All forces are measured in N
• PE mgh or N x h
• This is the same calculations shown to the right
  
• Both would result in 500 J of Potential Energy

10
Kinetic Energy

• It is the energy of motion possessed by moving
  objects
• It is a form of mechanical energy
• KE ½ m v2
• If you multiply the mass by 10 the KE increases
  by 10
• If you multiply the velocity by 10 the KE
  increases by 100
• Kinetic energy often appears in forms such as
  heat, sound, light and electricity and are over
  looked

11
Concept Check

• A car travels at 30 km/h and has kinetic energy
  of 1 MJ. If it travels twice as fast, how much
  kinetic energy will it have?
• If the v is multiplied by 2 it reflects 22 or 4
  times what it was before 4 MJ
• If it travels 3 times as fast, what will its KE
  be?
• 3 squared is 9 times or 9 MJ
• If it travels four times as fast, what will the
  KE be?
• 4 squared is 16 times or 16 MJ

12
Conservation of Energy

• Energy cannot be created or destroyed it may be
  transformed from one form to another or
  transferred from one object to another, but the
  total amount of energy never changes
• The transfers can become complex such as sunlight
  to stored sugar in plants to coal
• Another might be the suns energy being absorbed
  by water, water to rain, rain to rivers, rivers
  to dams and dams to electricity
• The Earths energy is generally traced back to
  the sun
• Another source is geothermal energy

13
Concept Check

• Does an automobile consumed more energy when the
  air conditioner is running, the radio is playing,
  or the lights are on?
• Yes, all require energy to work
• If there is a row of windmills, what happens to
  the winds speed before and after the power
  plant? Would it be windier before or after the
  plant?
• It would slow the wind by converting its KE into
  electrical energy. The side before the mills
  would be windier

14
Machines

• Machines either redirect force or multiple it
• If you could exclude friction, work input is
  equal to work output
• A common machine is the lever that consists of an
  arm and a fulcrum
• Archimedes once claimed that he could move the
  world if he had a lever long enough and a place
  to put the fulcrum

15
Concept Check

• If a lever is arranged so that input distance is
  twice the output distance, can we predict that
  energy output will be doubled?
• Energy can NEVER be changed because it is always
  conserved
• The resulting force would be doubled

16
Pulleys

• Redirecting or Multiplying force?

150 N Output
75 N Input
75 N Output
75 N Input
150 N Block
75 N Block
17
Efficiency

• Efficiency is a ratio
• Efficiency Work Done Output
• Energy Used Input
• Even pulleys have a little friction and heat is
  produced
• A car is very inefficient and we only get out a
  small fraction of the potential energy stored in
  fuel
• Where does the energy go?

18
Metabolism

• Our cells are like tiny machines that convert
  stored energy from food into kinetic energy
• The human body also has poor efficiency
• Why is this a good thing in humans but bad for
  engines?