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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.


Work and energy
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


Work and energy
Work

  • Can be calculated by the following equation

    • Work = force x distance (W= Fd)

      • W= work

      • F= force

      • d= distance


Work and energy
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