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Work and Energy. Chapter 12. Students Will Be Able To:. Explain the meaning of work. Explain how work and energy are related. Calculate work. Calculate power. Work.

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work and energy

Work and Energy

Chapter 12

students will be able to
Students Will Be Able To:
  • Explain the meaning of work.
  • Explain how work and energy are related.
  • Calculate work.
  • Calculate power.
slide3
Work
  • It is the transfer of energy to a body by the application of a force that causes the body to move in the direction of the force
  • It is only done when a force causes a change in the position or the motion of an object in the same direction of the applied force
    • Therefore, for work to occur, an object must move
slide4
Work
  • Can be calculated by the following equation
    • Work = force x distance (W= Fd)
      • W= work
      • F= force
      • d= distance
slide6
Work
  • Units for work include
    • N • m
    • J
      • Stands for joule
      • Is the main unit used for work
    • kg • m2/s2
  • 1J = 1 N • m= 1kg • m2/s2
example work
Example- Work
  • Imagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N?

W = Fd

= 190N x 2.0m

= 380 N • m

= 380 J

power
Power
  • It is a quantity that measures the rate at which work is done or energy is transformed
  • Can be calculated by the following equation
    • Power = work/time (P= W/t)
      • P= power
      • W= work
      • t= time
power1
Power
  • Units for power include
    • J/s
    • W
      • Stands for watt
      • Is the main unit used for power
  • 1W= 1J/s
example power
Example- Power
  • It takes 100 kJ of work to lift an elevator 18 m. If this is done in 20 s, what is the average power of the elevator during the process?
    • 100kJ x 1000J/1kJ = 100000 J
    • P = W/t

= 100000 J / 20 s

= 50000 J/s

= 50000 W

students will be able to1
Students Will Be Able To:
  • Explain how machines make work easier.
  • Explain how to calculate mechanical advantage.
  • Define efficiency.
  • Describe the six types of simple machines.
machines
Machines
  • Machines multiply and redirect forces
    • Help do work by redistributing the work that is put into them
    • Can also change the direction of the input force
    • Able to increase or decrease force by changing the distance over which the force is applied
  • Different forces can do the same amount of work
    • Due to decreasing the distance while increasing the force OR decreasing the force while increasing the distance
mechanical advantage
Mechanical Advantage
  • Mechanical advantage tells how much a machine multiplies force or increases distance
    • Calculated by the following equation
      • Input force is the force applied to the machine
      • Output force is the force applied by the machine to overcome resistance
mechanical advantage1
Mechanical Advantage
  • Amount of energy the machine transfers to the object cannot be greater than the amount of energy transferred to the machine
    • Some energy transferred is changed to heat due to friction
    • An ideal machine with no friction would have the same input work and output work
example mechanical advantage
Example- Mechanical Advantage
  • Calculate the mechanical advantage of a ramp that is 5.0 m long and 1.5 m high.
    • Input distance= 5.0 m and output distance= 1.5 m
    • Mechanical advantage = 5.0m/ 1.5m

= 3.33

efficiency
Efficiency
  • Is a measure of how much of the work put into a machine is changed into useful output work by the machine
  • Calculating efficiency
    • Efficiency = output work/input work X100%
  • Efficiency of a machine is always less than 100%
  • Lubricants can make machines more efficient by reducing friction
example efficiency
Example- Efficiency
  • A sailor uses a rope and an old, squeaky pulley to raise a sail that weighs 140 N. He finds that he must do 180 J of work on the rope in order to raise the sail by 1 m (doing 140 J of work on the sail). What is the efficiency of the pulley? Express your answer as a percentage.
    • Efficiency = output work/input work X100%

= 140 J/ 180 J X 100%

= 77.8%

simple machines
Simple Machines
  • One of the six basic types of machines, which are the basis for all other forms of machines
    • Divided into two families
      • Lever family
        • Simple lever
        • Pulley
        • Wheel and axle
      • Inclined plane family
        • Simple inclined place
        • Wedge
        • Screw
levers
Levers
  • Have a rigid arm that turns around a point called the fulcrum
    • Input arm is the part of the lever on which effort force is applied
    • Output arm is the part of the lever that exerts the resistance force
  • Divided into three classes based on the location of the fulcrum and of the input and output forces
first class levers
First Class Levers
  • Most common type
  • Fulcrum is located between the effort and resistance forces (input and output forces)
  • Multiplies and changes the direction of the force
  • Example- claw hammer, pliers
second class levers
Second Class Levers
  • Fulcrum is located at one end of the arm and the input force (effort force) is applied to the other end
  • Always multiplies force
  • Example- wheelbarrow, nutcrackers, hinged doors
third class levers
Third Class Levers
  • Effort force is between the output force (resistance force) and fulcrum
  • Responsible for multiplying distance rather than force
  • They have a mechanical advantage of less than 1
  • Example- human forearm
pulleys
Pulleys
  • Are modified levers
    • The point in the middle of a pulley is like the fulcrum of a lever
  • A single, fixed pulley has a mechanical advantage of 1
    • It is attached to something that doesn’t move
    • Force is not multiplied but direction is changed
  • A moveable pulley has one end of the rope fixed and the wheel free to move
    • It multiplies force
  • Multiple pulleys are sometimes put together in a single unit called a block and tackle
wheel and axles
Wheel and Axles
  • Is a machine with two wheels of different sizes rotating together
  • Is a lever or pulley connected to a shaft
    • The steering wheel of a car, screwdrivers, and cranks are common wheel-and-axel machines
inclined planes
Inclined Planes
  • Is a sloping surface that reduces the amount of force required to do work
  • Multiplies and redirects force
  • Turns a small input force into a large output force by spreading the work out over a large distance
    • Less force is required if a ramp is longer and less steep
wedge
Wedge
  • Is a modified inclined plane
  • Functions like two inclined planes back to back
  • Turns a single downward force into two forces directed out to the sides
  • Example- nails
screw
Screw
  • Is an inclined plane wrapped around a cylinder
  • Example- spiral staircases, jar lids
compound machines
Compound Machines
  • Is a machine made of more than one simple machine
  • Examples of compound machines
    • scissors, which use two first class levers joined at a common fulcrum
    • a car jack, which uses a lever in combination with a large screw
students will be able to2
Students Will Be Able To:
  • Explain the meaning of energy.
  • Distinguish between kinetic and potential energy.
  • Recognize the different ways that energy can be stored.
  • Calculate kinetic and potential energy.
energy
Energy
  • Energy is the ability to do work or to cause change
  • When you do work on an object, you transfer energy to that particular object
  • Whenever work is done, energy is transformed or transferred to another system
  • Energy is measured in joules
    • Work is also measured in joules
potential energy
Potential Energy
  • Is the energy that an object has because of the position, shape, or condition of the object
  • Is stored energy
  • 3 types exist
    • Chemical
      • Energy stored in chemical bonds between atoms
    • Elastic
      • Energy stored in stretched or compressed elastic materials
    • Gravitational
      • Energy stored in the gravitational field which exists between two or more objects
gravitational potential energy
Gravitational Potential Energy
  • Depends on mass, height, and acceleration due to gravity
  • Can be calculated by the following equation
    • PE = mass x free fall acceleration x height (PE = mgh)
      • PE= gravitational potential energy
      • m= mass
      • g= free fall acceleration (9.8 m/s2)
      • h= height
        • Usually measured from the ground but can also be measured from two objects
example gravitational pe
Example- Gravitational PE
  • A 65 kg rock climber ascends a cliff. What is the climber’s gravitational potential energy at a point 35 m above the base of the cliff?
    • PE = mgh

= 65 kg x 9.8 m/s2 x 35 m

= 22295 kg•m2/s2

= 22295 J

kinetic energy
Kinetic Energy
  • Is the energy of a moving object due to the object’s motion
  • Depends on mass and speed
    • Depends on speed more than mass
  • Can be calculated by the following equation
    • Kinetic Energy = ½ mass x speed squared

KE = ½ mv2

example ke
Example- KE
  • What is the kinetic energy of a 44 kg cheetah running at 31 m/s?
    • KE = ½ mv2

= ½ (44 kg)(31m/s)2

= 21142 kg•m/s2

= 21142 J

other forms of energy
Other Forms of Energy
  • The amount of work an object can do because of the object’s kinetic and potential energies is called mechanical energy
    • Mechanical energy is the sum of the potential energy and the kinetic energy in a system
  • In addition to mechanical energy, most systems contain non-mechanical energy
    • Non-mechanical energy does not usually affect systems on a large scale
other forms of energy1
Other Forms of Energy
  • Atoms and molecules have kinetic energy
    • The kinetic energy of particles is related to heat and temperature
  • Chemical reactions involve potential energy
    • The amount of chemical energy associated with a substance depends in part on the relative positions of the atoms it contains
  • Living things get energy from the sun
    • Plants use photosynthesis to turn the energy in sunlight into chemical energy
other forms of energy2
Other Forms of Energy
  • The sun gets energy from nuclear reactions
    • The sun is fueled by nuclear fusion reactions in its core
  • Electricity is a form of energy
    • Electrical energy is derived from the flow of charged particles, as in a bolt of lightning or in a wire
  • Light can carry energy across empty space
    • Light energy travels from the sun to Earth across empty space in the form of electromagnetic waves
energy transformations
Energy Transformations
  • Energy readily changes from one form to another
  • Potential energy can become kinetic energy
    • As a car goes down a hill on a roller coaster, potential energy changes to kinetic energy
  • Kinetic energy can become potential energy
    • The kinetic energy a car has at the bottom of a hill can do work to carry the car up another hill
energy transformations1
Energy Transformations
  • Mechanical energy can change to other forms of energy
    • Mechanical energy can change to non-mechanical energy as a result of friction, air resistance, or other means
students will be able to3
Students Will Be Able To:
  • Describe how energy is conserved when changing from one form to another.
  • Apply the law of conservation of energy to familiar situations.
the law of conservation of energy
The Law of Conservation of Energy
  • Energy may change from one form to another, but the total amount of energy never changes
    • Energy doesn’t appear out of nowhere
      • Whenever the total energy in a system increases, it must be due to energy that enters the system from an external source
    • Energy doesn’t disappear,but it can be changed to another form
energy systems
Energy Systems
  • Scientist study energy systems
    • Boundaries define a system
  • Systems may be open or closed
    • When the flow of energy into and out of a system is small enough that it can be ignored, the system is called a closed system
    • Most systems are open systems, which exchange energy with the space that surrounds them
mass energy
Mass  Energy
  • Mass as thought of as energy when discussing nuclear reactions
    • The total amount of mass and energy is conserved
  • 2 processes involved
    • Nuclear fusion
      • Two nuclei are fused together
    • Nuclear fission
      • Two nuclei are broken apart
food energy
Food Energy
  • Chemical potential energy from food that is stored in your body is used to fuel the processes that keep you alive
  • The food calorie is used to measure how much energy you get from various foods
  • One calorie is equivalent to 4180 J
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